Tiny doc bug: incorrect arg shown to dev_tools/modules.py in dev/modules.md

[mhucka]

Description of the issue

In the Cirq modules documentation, at the bottom, the command line to list modules is given as

python dev_tools/modules.py --list

but in fact, attempting to use --list will result in an error, because the argument should actually be list (without the dashes).

How to reproduce the issue

In a shell, with the current directory being the top of the Cirq repo, running python dev_tools/modules.py --list results in the following error:

usage: A utility for modules. [-h] {list,print_version,replace_version} ...
A utility for modules.: error: unrecognized arguments: --list

Cirq version

0.15.0.dev

[dstrain115]

Discussion: this has already been fixed.

[vtomole]

Fixed by #5638