Coreclr workflow instructions incorrect

[benaadams]

https://github.com/dotnet/runtime/blob/master/docs/workflow/building/coreclr/README.md

It no longer understands --subsetCategory

❯ .\build  --subsetCategory coreclr -configuration checked
MSBUILD : error MSB1001: Unknown switch.
Switch: --subsetCategory

For switch syntax, type "MSBuild -help"
Build failed.
Some builds failed:
        Configuration: checked, Architecture: x64

[benaadams]

Single dash now?

.\build -subsetCategory coreclr -configuration checked

[AaronRobinsonMSFT]

/cc @jkoritzinsky

[AaronRobinsonMSFT]

Fixed with #34663.