docs($interval): cancel() ignores falsy parameter

Closes angular#12552

Author: chrootsu

src/ng/interval.js

@@ -174,7 +174,7 @@ function $IntervalProvider() {
       * @description
       * Cancels a task associated with the `promise`.
       *
-      * @param {promise} promise returned by the `$interval` function.
+      * @param {Promise=} promise returned by the `$interval` function.
       * @returns {boolean} Returns `true` if the task was successfully canceled.
       */
     interval.cancel = function(promise) {