use valid SPDX identifier for license

Author: notslang

composer.json

@@ -2,7 +2,7 @@
   "name": "wp-sync-db/wp-sync-db-cli",
   "type": "wordpress-plugin",
   "homepage": "https://github.com/wp-sync-db/wp-sync-db-cli",
-  "license": "GPLv2",
+  "license": "GPL-2.0",
   "description": "WP Sync DB CLI Addon",
   "keywords": ["plugin","wordpress","wp-sync-db","cli"],
   "require": {