Update custom-404-page.md - fix a typo (#6218)
Update custom-404-page.md - fix a typo (#6218)

Merge pull request 6218
alex-kovac/jekyll,tillgrallert/jekyll,Strangehill/jekyll,jaybe-jekyll/jekyll,yhironaka/yhironaka.github.io,jekyll/jekyll,getandpost/jekyll.github.io,gencer/jekyll,gencer/jekyll,jekyll/jekyll,x-way/jekyll,chrisfinazzo/jekyll,alex-kovac/jekyll,gencer/jekyll,gencer/jekyll,alex-kovac/jekyll,ryanshaw/jekyll,jaybe-jekyll/jekyll,tillgrallert/jekyll,jaybe-jekyll/jekyll,Strangehill/jekyll,gencer/jekyll,ryanshaw/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,x-way/jekyll,x-way/jekyll,Strangehill/jekyll,chrisfinazzo/jekyll,getandpost/jekyll.github.io,tillgrallert/jekyll,fulldecent/jekyll,ryanshaw/jekyll,x-way/jekyll,Strangehill/jekyll,alex-kovac/jekyll,jaybe-jekyll/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,tillgrallert/jekyll,Strangehill/jekyll,alex-kovac/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,fulldecent/jekyll,jekyll/jekyll,fulldecent/jekyll