Fix wrong changelog code sample

I was testing this new feature of fetching the credentials from the command line and found out the code sample was wrong. Initially the PR was using `/` to navigate the credentials hierarchy but then @byroot changed it to use `.` instead.

Author: fredplante

railties/CHANGELOG.md

@@ -1,7 +1,7 @@
 *   Add command `rails credentials:fetch PATH` to get the value of a credential from the credentials file.
 
     ```bash
-    $ bin/rails credentials:fetch kamal_registry/password
+    $ bin/rails credentials:fetch kamal_registry.password
     ```
 
     *Matthew Nguyen*, *Jean Boussier*