[chore] [exporter/signalfx] Update trace correlation docs (open-telemetry#36866)

Don't mention SAPM exporter since it's going to be deprecated. It
doesn't matter which exporter is used to send traces.

Author: dmitryax

exporter/signalfxexporter/README.md

@@ -150,7 +150,7 @@ Information about queued retry configuration parameters can be found
 
 ## Traces Configuration (correlation only)
 
-:warning: _Note that traces must still be sent in using [sapmexporter](../sapmexporter) to see them in SignalFx._
+:warning: _Note that traces must still be sent to Splunk Observability via a separate trace exporter._
 
 When traces are sent to the signalfx exporter it correlates traces to metrics. When a new service or environment is
 seen it associates the source (e.g. host or pod) to that service or environment in SignalFx. Metrics can then be