Docs: Correct @param type in fix_phpmailer_messageid().

Follow-up to [48033].

Props justlevine.
See #52217, #62281.

git-svn-id: https://develop.svn.wordpress.org/trunk@59334 602fd350-edb4-49c9-b593-d223f7449a82

src/wp-includes/ms-functions.php

@@ -2273,7 +2273,7 @@ function add_new_user_to_blog( $user_id, $password, $meta ) {
  *
  * @since MU (3.0.0)
  *
- * @param PHPMailer $phpmailer The PHPMailer instance (passed by reference).
+ * @param PHPMailer\PHPMailer\PHPMailer $phpmailer The PHPMailer instance (passed by reference).
  */
 function fix_phpmailer_messageid( $phpmailer ) {
 	$phpmailer->Hostname = get_network()->domain;