Keep RedirectController links consistent.

There are two links defined to RedirectController, with backticks and
without them.

This commit removes the link without backticks for the sake of
consistency as discussed here: https://github.com/rails/rails/pull/42703/files#r667270569

Author: ceritium

guides/source/active_storage_overview.md

@@ -586,9 +586,8 @@ require a higher level of protection consider implementing
 To generate a permanent URL for a blob, you can pass the blob to the
 [`url_for`][ActionView::RoutingUrlFor#url_for] view helper. This generates a
 URL with the blob's [`signed_id`][ActiveStorage::Blob#signed_id]
-that is routed to the blob's [`RedirectController`][ActiveStorage::Blobs::RedirectController]
+that is routed to the blob's [`RedirectController`][`ActiveStorage::Blobs::RedirectController`]
 
-[ActiveStorage::Blobs::RedirectController]: https://github.com/rails/rails/blob/main/activestorage/app/controllers/active_storage/blobs/redirect_controller.rb
 ```ruby
 url_for(user.avatar)
 # => /rails/active_storage/blobs/:signed_id/my-avatar.png