Update/fix a few lines in taxi example readme

[DABH]

1. On my mac (python 3.8.2, etc.), the command was virtualenv .venv. Googling around, it seems like virtualenv is more common... is venv an alias you defined on your machine or is that the right command name for some platforms?

2. Looks like speed is defined as a positional not named argument in the simulation script, this was a necessary fix to make the script run

Thanks!