-runtimeConfiguration breaks -r as a shortcut for restore on build.ps1

[billwert]

PS E:\source\runtime [master ≡] > .\build.cmd -r
E:\source\runtime\eng\build.ps1 : Missing an argument for parameter 'runtimeConfiguration'. Specify a parameter of type 'System.String' and try again.
    + CategoryInfo          : InvalidArgument: (:) [build.ps1], ParentContainsErrorRecordException
    + FullyQualifiedErrorId : MissingArgument,build.ps1

This previously would run restore. Passing -restore works fine. Trivial fix is to just clean up the help text.

[safern]

cc: @ViktorHofer this is due to PS not letting 2 arguments start with the same substring.