docs: fix type for aggregation cursor unwind param (#2636)

NODE-2895

Author: climba03003

lib/aggregation_cursor.js

@@ -202,7 +202,7 @@ class AggregationCursor extends Cursor {
   /**
    * Add a unwind stage to the aggregation pipeline
    * @method
-   * @param {number} field The unwind field name.
+   * @param {(string|object)} field The unwind field name or stage document.
    * @return {AggregationCursor}
    */
   unwind(field) {