Remove stale documentation about assets.debug

Remove stale documentation about `assets.debug`
since the default was removed from the `development.rb` template:
rails@adec7e7

Author: grantbdev

guides/source/configuring.md

@@ -695,7 +695,7 @@ Enables the use of SHA256 fingerprints in asset names. Set to `true` by default.
 
 #### `config.assets.debug`
 
-Disables the concatenation and compression of assets. Set to `true` by default in `development.rb`.
+Disables the concatenation and compression of assets.
 
 #### `config.assets.version`