Adjusting WCF performance counters descritpion (Calls Duration)

[ptylenda]

Summary

While working on Prometheus integration for WCF services monitoring, I have noticed that https://github.com/dotnet/docs/blob/ee33cc88df5e22a9a8ff6d99b511a642089d38a1/docs/framework/wcf/diagnostics/performance-counters/toc.md has wrong performance counters names for "Calls Duration". Raw output of TypePerf.exe -qx reveals that naming is as following:

\ServiceModelEndpoint 4.0.0.0(xxx@xxx)\Calls Duration
\ServiceModelOperation 4.0.0.0(xxx@xxx)\Calls Duration
\ServiceModelService 4.0.0.0(xxx@xxx)\Calls Duration

Whereas the docs state that it is "Call Duration" (note the missing "s") for all categories.

Note: I am not able to verify if this is specific for a given version of .NET framework, or docs had wrong naming since the beginnig.

[dnfclas]

All CLA requirements met.

[rpetrusha]

Thank you, @ptylenda, for contributing to the dotnet/docs repo and for noticing that the performance counter name is incorrect. We'll merge your PR now. Your changes should appear live on docs.microsoft.com in the next day or two.

[ptylenda]

@rpetrusha Thanks!