Update wpmdb.php

[tditlu]

Fix path to "asset/dist/css/styles.css" not being correct if plugin is placed in another folder, e.g. putting the plugin in mu-plugins folder with https://roots.io/bedrock/docs/mu-plugins-autoloader/ or https://github.com/wordplate/autoloader

[tditlu]

You are already using this technique to get the asset dir for the "asset/dist/img/wp-migrate-db-pro.jpg" image in "template/wpmdb/sidebar.php" (line 3): https://github.com/deliciousbrains/wp-migrate-db/blob/master/template/wpmdb/sidebar.php#L3