deprecator.rb: fix typo for --serve command (#6229)
deprecator.rb: fix typo for --serve command (#6229)

Merge pull request 6229
ryanshaw/jekyll,gencer/jekyll,Strangehill/jekyll,ryanshaw/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,tillgrallert/jekyll,ryanshaw/jekyll,jekyll/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,x-way/jekyll,alex-kovac/jekyll,jaybe-jekyll/jekyll,fulldecent/jekyll,fulldecent/jekyll,gencer/jekyll,ryanshaw/jekyll,getandpost/jekyll.github.io,jekyll/jekyll,tillgrallert/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,tillgrallert/jekyll,alex-kovac/jekyll,alex-kovac/jekyll,fulldecent/jekyll,x-way/jekyll,jaybe-jekyll/jekyll,x-way/jekyll,fulldecent/jekyll,gencer/jekyll,jekyll/jekyll,chrisfinazzo/jekyll,chrisfinazzo/jekyll,Strangehill/jekyll,tillgrallert/jekyll,Strangehill/jekyll,gencer/jekyll,jekyll/jekyll,alex-kovac/jekyll,Strangehill/jekyll,gencer/jekyll,chrisfinazzo/jekyll,getandpost/jekyll.github.io,yhironaka/yhironaka.github.io,getandpost/jekyll.github.io,jaybe-jekyll/jekyll