Fix AMo code example indent, add missing author to entry [ci skip]

activemodel/CHANGELOG.md

@@ -5,11 +5,13 @@
 
     Example:
 
-        # given User has_secure_password.
+        # Given User has_secure_password.
         @user.password = ""
         @user.password_confirmation = ""
         @user.valid?(:update) # used to be false
 
+    *Yves Senn*
+
 *   `validates_confirmation_of` does not override writer methods for
     the confirmation attribute if no reader is defined.