Coding Standards: Use more meaningful variable names in Custom Image Header.

Per the [https://developer.wordpress.org/coding-standards/wordpress-coding-standards/php/#naming-conventions Naming Conventions]:

> Don’t abbreviate variable names unnecessarily; let the code be unambiguous and self-documenting.

Follow-up to [27497], [27849].

Props costdev, mukesh27.
See #63168.

git-svn-id: https://develop.svn.wordpress.org/trunk@60970 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-custom-image-header.php

@@ -1547,8 +1547,8 @@ public function get_default_header_images() {
 
 		$already_has_default = false;
 
-		foreach ( $this->default_headers as $k => $h ) {
-			if ( $h['url'] === $default ) {
+		foreach ( $this->default_headers as $k => $header ) {
+			if ( $header['url'] === $default ) {
 				$already_has_default = true;
 				break;
 			}