This document and assocaited source code need updating, since --wasmMainJS and --customruntimepack options are no longer needed, and the --runtimeSrcDir argument is now required:
https://github.com/dotnet/BenchmarkDotNet/blob/master/docs/articles/samples/IntroWasm.md/#L1

I intend to take on this task; creating the issue so I can keep track of things.