fix: update repository links to correct tracing example URLs

- SnapTrace: github.com/getsentry/snaptrace-tracing-example
- NullFlix: github.com/getsentry/nullflix-tracing-example
- Both repositories are now accessible and contain comprehensive tracing examples

Author: codyde

docs/platforms/javascript/common/tracing/span-metrics/examples.mdx

@@ -169,7 +169,7 @@ What to monitor with span metrics:
 
 ## Media Upload with Background Processing (React + Express)
 
-Example Repository: [SnapTrace](https://github.com/getsentry/snaptrace)
+Example Repository: [SnapTrace](https://github.com/getsentry/snaptrace-tracing-example)
 
 **Challenge:** Track user-perceived upload time, server-side validation, and async media processing (optimization, thumbnail generation) while maintaining trace continuity across async boundaries.
 
@@ -376,7 +376,7 @@ What to monitor with span metrics:
 
 ## Search Autocomplete (debounced, cancellable, performance monitoring)
 
-Example Repository: [NullFlix](https://github.com/getsentry/nullflix-tracing-sample)
+Example Repository: [NullFlix](https://github.com/getsentry/nullflix-tracing-example)
 
 **Challenge:** Users type quickly in search; you need to debounce requests, cancel in-flight calls, handle errors gracefully, and monitor performance across different query types while keeping latency predictable.