Apply suggestion from @westonruter

Co-authored-by: Weston Ruter <[email protected]>

Author: lloc

src/wp-admin/network/users.php

@@ -86,7 +86,8 @@
 											/* translators: %s: User login. */
 											__( 'Warning! User cannot be modified. The user %s is a network administrator.' ),
 											esc_html( $user->user_login )
-										)
+										),
+										403
 									);
 								}