Fix a typo in the description for the `_wp_filter_build_unique_id()` DocBlock.

Props lamosty.
Fixes #31732.

Built from https://develop.svn.wordpress.org/trunk@31859


git-svn-id: http://core.svn.wordpress.org/trunk@31838 1a063a9b-81f0-0310-95a4-ce76da25c4cd

wp-includes/plugin.php

@@ -839,7 +839,7 @@ function _wp_call_all_hook($args) {
  * Build Unique ID for storage and retrieval.
  *
  * The old way to serialize the callback caused issues and this function is the
- * solution. It works by checking for objects and creating an a new property in
+ * solution. It works by checking for objects and creating a new property in
  * the class to keep track of the object and new objects of the same class that
  * need to be added.
  *