Docs: Add missing @param type for wp_cache_get_last_changed().

Props afzalmultani, Soean.
Fixes #39571.

git-svn-id: https://develop.svn.wordpress.org/trunk@39900 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/functions.php

@@ -5626,7 +5626,7 @@ function wp_generate_uuid4() {
  *
  * @since 4.7.0
  *
- * @param $group Where the cache contents are grouped.
+ * @param string $group Where the cache contents are grouped.
  *
  * @return string $last_changed UNIX timestamp with microseconds representing when the group was last changed.
  */