Fixed VSTest doc incorrect dotnet switch reference

[mikey-t]

Fixed a mistake in the doc Documentation\VSTestIntegration.md.

Wrong:

[-r|--results-directory]

Correct:

--results-directory

The -r switch is a valid switch, but it's the short version of the --runtime switch, not a short version of the --results-directory switch:

dotnet test --help
...
  --results-directory <RESULTS_DIR>                          The directory where the test results will be placed.
...
  -r, --runtime <RUNTIME_IDENTIFIER>                         The target runtime to test for.
...

[mikey-t]

@dotnet-policy-service agree