Add header demarcation to Action Cable guide
Add header demarcation to Action Cable guide

Guide generation is configured to detect a guide header by a 40-or-more-hyphen long line. The Action Cable guide was missing this line, so the page title on the guides site was only showing "Ruby on Rails Guides", and the section title in the Kindle index was "Ruby on Rails Guides". Adding the header demarcation line back in fixes both of these.

Fixes #25697

[ci skip]

esparta/rails,tgxworld/rails,yawboakye/rails,illacceptanything/illacceptanything,koic/rails,felipecvo/rails,georgeclaghorn/rails,betesh/rails,mathieujobin/reduced-rails-for-travis,lucasmazza/rails,Envek/rails,prathamesh-sonpatki/rails,repinel/rails,alecspopa/rails,kachick/rails,repinel/rails,jeremy/rails,ttanimichi/rails,eileencodes/rails,pvalena/rails,MSP-Greg/rails,felipecvo/rails,gauravtiwari/rails,bolek/rails,illacceptanything/illacceptanything,illacceptanything/illacceptanything,rossta/rails,utilum/rails,sealocal/rails,pschambacher/rails,elfassy/rails,kenta-s/rails,Spin42/rails,pvalena/rails,schuetzm/rails,kmcphillips/rails,untidy-hair/rails,samphilipd/rails,odedniv/rails,palkan/rails,baerjam/rails,georgeclaghorn/rails,pvalena/rails,lucasmazza/rails,lcreid/rails,aditya-kapoor/rails,joonyou/rails,palkan/rails,iainbeeston/rails,mechanicles/rails,matrinox/rails,illacceptanything/illacceptanything,gauravtiwari/rails,illacceptanything/illacceptanything,assain/rails,mijoharas/rails,mtsmfm/railstest,illacceptanything/illacceptanything,mohitnatoo/rails,kmayer/rails,rossta/rails,tjschuck/rails,vipulnsward/rails,bradleypriest/rails,travisofthenorth/rails,maicher/rails,Vasfed/rails,MSP-Greg/rails,esparta/rails,lcreid/rails,prathamesh-sonpatki/rails,rails/rails,tjschuck/rails,yhirano55/rails,palkan/rails,lcreid/rails,shioyama/rails,koic/rails,mechanicles/rails,rbhitchcock/rails,kirs/rails-1,xlymian/rails,untidy-hair/rails,yasslab/railsguides.jp,sealocal/rails,hanystudy/rails,arjes/rails,riseshia/railsguides.kr,pschambacher/rails,deraru/rails,MichaelSp/rails,EmmaB/rails-1,tjschuck/rails,yalab/rails,illacceptanything/illacceptanything,kmayer/rails,deraru/rails,yhirano55/rails,illacceptanything/illacceptanything,Erol/rails,kenta-s/rails,rbhitchcock/rails,arjes/rails,Stellenticket/rails,rbhitchcock/rails,bogdanvlviv/rails,brchristian/rails,yahonda/rails,stefanmb/rails,starknx/rails,matrinox/rails,jeremy/rails,MichaelSp/rails,joonyou/rails,notapatch/rails,georgeclaghorn/rails,illacceptanything/illacceptanything,gavingmiller/rails,ledestin/rails,ngpestelos/rails,kamipo/rails,Spin42/rails,baerjam/rails,illacceptanything/illacceptanything,lucasmazza/rails,mathieujobin/reduced-rails-for-travis,gcourtemanche/rails,matrinox/rails,mohitnatoo/rails,starknx/rails,ttanimichi/rails,mechanicles/rails,hanystudy/rails,mijoharas/rails,stefanmb/rails,hanystudy/rails,mtsmfm/rails,BlakeWilliams/rails,gfvcastro/rails,pschambacher/rails,maicher/rails,utilum/rails,Stellenticket/rails,kamipo/rails,kmcphillips/rails,gfvcastro/rails,ngpestelos/rails,yhirano55/rails,mathieujobin/reduced-rails-for-travis,flanger001/rails,joonyou/rails,shioyama/rails,mohitnatoo/rails,kmcphillips/rails,mohitnatoo/rails,gcourtemanche/rails,BlakeWilliams/rails,tgxworld/rails,illacceptanything/illacceptanything,xlymian/rails,ledestin/rails,MSP-Greg/rails,rafaelfranca/omg-rails,riseshia/railsguides.kr,mijoharas/rails,marklocklear/rails,voray/rails,Erol/rails,voray/rails,Envek/rails,lcreid/rails,riseshia/railsguides.kr,coreyward/rails,bradleypriest/rails,MichaelSp/rails,Sen-Zhang/rails,tjschuck/rails,aditya-kapoor/rails,Vasfed/rails,kaspth/rails,palkan/rails,maicher/rails,odedniv/rails,kmcphillips/rails,koic/rails,yawboakye/rails,bolek/rails,samphilipd/rails,schuetzm/rails,Edouard-chin/rails,Vasfed/rails,fabianoleittes/rails,mtsmfm/rails,ngpestelos/rails,Spin42/rails,Edouard-chin/rails,arjes/rails,travisofthenorth/rails,kaspth/rails,yhirano55/rails,arunagw/rails,yahonda/rails,untidy-hair/rails,BlakeWilliams/rails,kmayer/rails,samphilipd/rails,kenta-s/rails,sergey-alekseev/rails,vipulnsward/rails,notapatch/rails,repinel/rails,shioyama/rails,kddeisz/rails,illacceptanything/illacceptanything,Erol/rails,richseviora/rails,iainbeeston/rails,mechanicles/rails,printercu/rails,kddeisz/rails,xlymian/rails,Erol/rails,marklocklear/rails,kachick/rails,assain/rails,felipecvo/rails,Stellenticket/rails,richseviora/rails,sealocal/rails,rails/rails,kirs/rails-1,arunagw/rails,Envek/rails,travisofthenorth/rails,tgxworld/rails,printercu/rails,tgxworld/rails,EmmaB/rails-1,marklocklear/rails,kaspth/rails,yahonda/rails,iainbeeston/rails,yasslab/railsguides.jp,iainbeeston/rails,bogdanvlviv/rails,elfassy/rails,fabianoleittes/rails,esparta/rails,gavingmiller/rails,ttanimichi/rails,starknx/rails,flanger001/rails,EmmaB/rails-1,eileencodes/rails,betesh/rails,kachick/rails,odedniv/rails,repinel/rails,gcourtemanche/rails,schuetzm/rails,kddeisz/rails,travisofthenorth/rails,coreyward/rails,printercu/rails,brchristian/rails,Envek/rails,assain/rails,aditya-kapoor/rails,notapatch/rails,mtsmfm/railstest,bradleypriest/rails,Vasfed/rails,illacceptanything/illacceptanything,joonyou/rails,Edouard-chin/rails,yawboakye/rails,stefanmb/rails,deraru/rails,notapatch/rails,flanger001/rails,bolek/rails,coreyward/rails,gauravtiwari/rails,kamipo/rails,mtsmfm/rails,baerjam/rails,betesh/rails,fabianoleittes/rails,rossta/rails,yalab/rails,illacceptanything/illacceptanything,tijwelch/rails,yawboakye/rails,baerjam/rails,raf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