Fixed incorrect documentation for mui-slide function

Note that the `$amount` parameter is missing documentation.

src/transitions/_slide.scss

@@ -2,9 +2,9 @@
 /// @param {Keyword} $state [in] - State to transition to.
 /// @param {Keyword} $direction [left] - Side of the element to slide from. Can be `top`, `right`, `bottom`, or `left`.
 /// @param {Boolean} $fade [false] - Set to `true` to fade the element in or out simultaneously.
-/// @param {Keyword} $duration [500ms] - Length (speed) of the transition.
-/// @param {Keyword|Function} $timing [linear] - Easing of the transition.
-/// @param {Number} $delay [0s] - Delay in seconds or milliseconds before the transition starts.
+/// @param {Duration} $duration [null] - Length (speed) of the transition.
+/// @param {Keyword|Function} $timing [null] - Easing of the transition.
+/// @param {Duration} $delay [null] - Delay in seconds or milliseconds before the transition starts.
 @mixin mui-slide (
   $state: in,
   $direction: left,