Docs: Standardize references to "meta box" or "meta boxes" as two distinct words throughout core documentation per the core spelling guide.

See #32246.


git-svn-id: https://develop.svn.wordpress.org/trunk@38029 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-admin/includes/post.php

@@ -1166,7 +1166,7 @@ function wp_edit_attachments_query( $q = false ) {
 }
 
 /**
- * Returns the list of classes to be used by a metabox
+ * Returns the list of classes to be used by a meta box.
  *
  * @since 2.5.0
  *