Remove keyPath as a possible return value on the event object for onDidChange in config.coffee

The value is never returned on the object so the documention was updated to reflect that

src/config.coffee

@@ -422,7 +422,6 @@ class Config
   #   * `event` {Object}
   #     * `newValue` the new value of the key
   #     * `oldValue` the prior value of the key.
-  #     * `keyPath` the keyPath of the changed key
   #
   # Returns a {Disposable} with the following keys on which you can call
   # `.dispose()` to unsubscribe.