fix: replace npmjs link with GitHub repo link

npmjs.com returns 403 to automated link checkers. Link to the
OpenTelemetry GitHub repo instead.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: bartlomieju

docs/latest/advanced/opentelemetry.md

@@ -145,7 +145,7 @@ When an OpenTelemetry exporter is active, Fresh automatically injects a
 ```
 
 This allows client-side OpenTelemetry instrumentation (such as
-[`@opentelemetry/instrumentation-document-load`](https://www.npmjs.com/package/@opentelemetry/instrumentation-document-load))
+[`@opentelemetry/instrumentation-document-load`](https://github.com/open-telemetry/opentelemetry-js-contrib/tree/main/plugins/web/opentelemetry-instrumentation-document-load))
 to link browser performance traces back to the server-side span that rendered
 the page, giving you end-to-end visibility from server rendering through page
 load.