fix problems.schema.json regex for UUIDs

spec/problems.schema.json

@@ -21,7 +21,7 @@
       "instance": {
         "description": "Betroffener Teil der Übertragung. Mögliche Werte: `data`, `metadata`, `attachment:` + UUID des Attachments, `reply`, `submission` oder `other`.",
         "type": "string",
-        "pattern": "^(data|metadata|attachment:[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[4][0-9A-Fa-f]{3}-[89AB][0-9A-Fa-f]{3}-[0-9A-Fa-f]{12}|reply|submission|other)$"
+        "pattern": "^(data|metadata|attachment:[0-9A-Fa-f]{8}-[0-9A-Fa-f]{4}-[4][0-9A-Fa-f]{3}-[89ABab][0-9A-Fa-f]{3}-[0-9A-Fa-f]{12}|reply|submission|other)$"
       }
     },
     "required": [