docs($rootScope): add missing round bracket

Closes #15299

Author: oddmj

src/ng/rootScope.js

@@ -285,7 +285,7 @@ function $RootScopeProvider() {
        *   $digest()} and should return the value that will be watched. (`watchExpression` should not change
        *   its value when executed multiple times with the same input because it may be executed multiple
        *   times by {@link ng.$rootScope.Scope#$digest $digest()}. That is, `watchExpression` should be
-       *   [idempotent](http://en.wikipedia.org/wiki/Idempotence).
+       *   [idempotent](http://en.wikipedia.org/wiki/Idempotence).)
        * - The `listener` is called only when the value from the current `watchExpression` and the
        *   previous call to `watchExpression` are not equal (with the exception of the initial run,
        *   see below). Inequality is determined according to reference inequality,