Fix grammar in Caching with Rails docs

[elainefang]

No description provided.

[pixeltrix]

Would If you want to cache a fragment under certain conditions, you can use be better because you can do things like:

<% cache_if active? && admin?, :navbar do %>

[elainefang]

Yes, that does work better. I updated the file.

[vijaydev]

Can you please squash the two commits to one? And please add [ci skip] in the commit message so that Travis doesn't run a build for this.

[elainefang]

Sure. I just updated the commit.