[ci skip] Close the meta tag with '/>' instead of '>'

This change fixes the 42 instances of `end tag for "meta" omitted, but
OMITTAG NO was specified` error message that comes up when the full
railsguides are validated.

Full error list at
https://gist.github.com/prakashmurthy/9086679.
Merge pull request #14107 from prakashmurthy/patch-1

Close the meta tag with '/>' instead of '>'