fix typo

Author: mydea

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

@@ -70,7 +70,7 @@ For detailed examples of how to implement span metrics in common scenarios, see
 
 ## Adding Metrics to All Spans
 
-To consistently add metrics across all spans in your application, you can use the `beforeSendTransaction` callback:
+To consistently add metrics across all spans in your application, you can use the `beforeSendSpan` callback:
 
 ```javascript
 Sentry.init({