[connector/spanmetrics] CHANGELOG is fixed to reflect the correct fea… (

#35852)

<!--Ex. Fixing a bug - Describe the bug and how this fixes the issue.
Ex. Adding a feature - Explain what this achieves.-->
#### Description
The wrong feature gate was exposed in the CHANGELOG, this has been
corrected to the correct feature gate
'connector.spanmetrics.legacyMetricNames'

<!-- Issue number (e.g. #1234) or full URL to issue, if applicable. -->
#### Link to tracking issue
Fixes #35766 

<!--Describe the documentation added.-->
#### Documentation
CHANGELOG.md is updated to have the correct feature gate name.
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CHANGELOG.md

@@ -368,7 +368,7 @@ arrow.waiter_limit -> admission.waiter_limit
   https://github.com/open-telemetry/opentelemetry-collector/pull/6250
 
 - `spanmetricsconnector`: Improve consistency between metrics generated by spanmetricsconnector. Added traces.span.metrics as default namespace (#33227, #32818)
-  Default namespace for the generated metrics is traces.span.metrics now. | The deprecated metrics are: calls, duration and events. | The feature flag connector.spanmetrics.legacyLatencyMetricNames was added to revert the behavior.
+  Default namespace for the generated metrics is traces.span.metrics now. | The deprecated metrics are: calls, duration and events. | The feature flag connector.spanmetrics.legacyMetricNames was added to revert the behavior.
 - `servicegraphconnector`: Fix histogram metrics miss unit (#34511)
   All metrics will remove the suffix `_seconds`. It will not introduce breaking change if users use | `prometheusexporter` or `prometheusremotewriteexporter` to exporter metrics in pipeline. | In some cases, like using `clickhouseexporter`(save data in native OTLP format), it will be a breaking change. | Users can use `transformprocessor` to add back this suffix.
 - `gitproviderreceiver`: The Git Provider Receiver has been renamed to GitHub Receiver. (#34731)