Fix Collection@firstWhere() return docblock (laravel#25988)

Like Collection@first(), this method will return
an item in the collection (or null), not an
instance of the collection.

src/Illuminate/Support/Collection.php

@@ -677,7 +677,7 @@ public function first(callable $callback = null, $default = null)
      * @param  string  $key
      * @param  mixed  $operator
      * @param  mixed  $value
-     * @return static
+     * @return mixed
      */
     public function firstWhere($key, $operator, $value = null)
     {