docs($q): fixed grammatical error

"Promises" should be of the possessive form.

Closes #6082

src/ng/q.js

@@ -73,7 +73,7 @@
  *   constructed via `$q.reject`, the promise will be rejected instead.
  * - `reject(reason)` – rejects the derived promise with the `reason`. This is equivalent to
  *   resolving it with a rejection constructed via `$q.reject`.
- * - `notify(value)` - provides updates on the status of the promises execution. This may be called
+ * - `notify(value)` - provides updates on the status of the promise's execution. This may be called
  *   multiple times before the promise is either resolved or rejected.
  *
  * **Properties**