Fix Layer.mask parameter and return type documentation

Corrected the parameter description from 'Effect name or index' to 'Mask name or index' and updated the return type from Effect to Mask in the sub-objects documentation.

Author: nsoarim

docs/layer/sub-objects.md

@@ -97,14 +97,14 @@ The `index` value will have After Effects finds the mask by its index in the Tim
 +-----------+--------+------------------------------+
 | Parameter |  Type  |         Description          |
 +===========+========+==============================+
-| `name`    | String | Effect name or index to get. |
+| `name`    | String | Mask name or index to get.   |
 |           |        |                              |
 | `index`   | Number |                              |
 +-----------+--------+------------------------------+
 
 #### Returns
 
-[Effect](../objects/effect.md)
+[Mask](../objects/mask.md)
 
 #### Example