Documentation and code don't agree on the flag for number of executions

[glandium]

The code uses --iterations (and iterations for the marker), but the README says --executions/executions. I'm not directly submitting a PR because I'm not sure which one you'd want.

While here, the README should probably make it clear that the flag is the total number of executions of the test, not the number of times the test runs on a single thread.