update doc about assets digest class [ci skip]

The default digest class changed to SHA256 in sprockets 3.0.
Ref: https://github.com/sstephenson/sprockets/pull/647

guides/source/configuring.md

@@ -175,11 +175,11 @@ pipeline is enabled. It is set to `true` by default.
 
 * `config.assets.manifest` defines the full path to be used for the asset precompiler's manifest file. Defaults to a file named `manifest-<random>.json` in the `config.assets.prefix` directory within the public folder.
 
-* `config.assets.digest` enables the use of MD5 fingerprints in asset names. Set to `true` by default.
+* `config.assets.digest` 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`.
 
-* `config.assets.version` is an option string that is used in MD5 hash generation. This can be changed to force all files to be recompiled.
+* `config.assets.version` is an option string that is used in SHA256 hash generation. This can be changed to force all files to be recompiled.
 
 * `config.assets.compile` is a boolean that can be used to turn on live Sprockets compilation in production.