Update telemetry doc to refer to depecate "preview"

docs/telemetry.md

@@ -8,7 +8,7 @@ By using the Glean SDK, Fenix can send the pings the SDK owns and defines, as do
 Additional metrics or pings defined by Fenix are documented in the [Glean SDK autogenerated docs](metrics.md).
 
 ## Leanplum
-See [here](https://github.com/mozilla-mobile/fenix/blob/master/docs/mma.md) for details on Leanplum usage in Firefox Preview.
+See [here](https://github.com/mozilla-mobile/fenix/blob/master/docs/mma.md) for details on Leanplum usage in Firefox for Android.
 
 ## Crash reporting
-See [here](https://github.com/mozilla-mobile/fenix/blob/master/docs/crash-reporting.md) for details on crash reporting in Firefox Preview.
+See [here](https://github.com/mozilla-mobile/fenix/blob/master/docs/crash-reporting.md) for details on crash reporting in Firefox for Android.