guides/source/caching_with_rails.md: remove dup

The deleted line initially had single quotes, but was changed in this PR:
https://github.com/rails/rails/pull/53459/files#diff-3673de0675cbb963764640c2874d899ff36ce0ebd1f7c19d8ed87cfa431ef5b6R218

I'd argue it didn't add much initially, but it's now a duplcate and is
confusing.

Author: pjambet

guides/source/caching_with_rails.md

@@ -299,7 +299,6 @@ how to decode:
 ```ruby
 render partial: "comments/comment", collection: commentable.comments
 render "comments/comments"
-render "comments/comments"
 render("comments/comments")
 
 render "header" # translates to render("comments/header")