[chore] [receiver/signalfx] Update README.md (open-telemetry#30748)

Fixing link.

**Description:** <Describe what has changed.>
<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->

**Link to tracking Issue:** <Issue number if applicable>

**Testing:** <Describe what testing was performed and which tests were
added.>

**Documentation:** <Describe the documentation added.>

receiver/signalfxreceiver/README.md

@@ -22,7 +22,7 @@ format](https://github.com/signalfx/com_signalfx_metrics_protobuf).
 format](https://github.com/signalfx/com_signalfx_metrics_protobuf/blob/master/proto/signalfx_metrics.proto#L137).
 More information about sending custom events can be found in the [SignalFx
 Developers
-Guide](https://developers.signalfx.com/ingest_data_reference.html#tag/Send-Custom-Events).
+Guide](https://dev.splunk.com/observability/reference/api/ingest_data/latest).
 
 ## Configuration