Fix servicenow-sdk ref: ServiceNow/sdk uses master, not main

The ServiceNow/sdk repository's default branch is 'master' and there is
no 'main' branch. The pinned SHA (06adf37) is the current head of
'master'. Update the ref so future SHA bumps target the correct branch.

.claude-plugin/marketplace.json

@@ -1819,7 +1819,7 @@
         "source": "git-subdir",
         "url": "https://github.com/ServiceNow/sdk.git",
         "path": "providers/claude/plugin",
-        "ref": "main",
+        "ref": "master",
         "sha": "06adf37ca78c270a57f93e7b9dfbb7bf16e24611"
       },
       "homepage": "https://servicenow.github.io/sdk/"