Fix wrong license SBDX classifier

Duh, phpBB.com is *so* annoying...

gwynethllewelyn/postlocalstorage/composer.json

@@ -4,7 +4,7 @@
 	"description": "A very simple phpBB3 extension to locally store the content of a post being written, to avoid losing everything after a crash or session expiration.",
 	"homepage": "https://github.com/GwynethLlewelyn/post-local-storage",
 	"version": "1.0.2",
-	"license": "gpl-2-0-only",
+	"license": "GPL-2.0-only",
 	"keywords": [
 		"phpbb",
 		"extension",