Fix plugin schema URL to point to main (#3829)

# What this PR does

Changes the schema url to point to main so that it has latest changes
(roles section is defined for instance)

## Which issue(s) this PR fixes

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

grafana-plugin/src/plugin.json

@@ -1,5 +1,5 @@
 {
-  "$schema": "https://raw.githubusercontent.com/grafana/grafana/master/docs/sources/developers/plugins/plugin.schema.json",
+  "$schema": "https://raw.githubusercontent.com/grafana/grafana/main/docs/sources/developers/plugins/plugin.schema.json",
   "type": "app",
   "name": "Grafana OnCall",
   "id": "grafana-oncall-app",