Corrected the instructions for running in parallel

[rwest]

The option -n $Processes is an argument to rmg.py not to python.

Motivation or Problem

The documentation suggested python -n $Processes path/to/rmg.py path/to/input.py

Description of Changes

It now says python path/to/rmg.py -n $Processes path/to/input.py

[ajocher]

Ran superminimal and minimal examples with the updated documentation and both examples worked as expected with multiprocessing.

[mliu49]

Please don't merge yet! I haven't decided exactly how we'll handle the PRs which have been opened since Py3 stuff started.

[rwest]

Right.
I presume it can be similar to ones that were already open but not merged? I.e we'll rebase them. But perhaps pick some bugfix ones to backport if we have a maintenance branch of RMG 2.4 for a little while?
Anyway, needn't be discussed here

[codecov]

Codecov Report

Merging #1709 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1709   +/-   ##
=======================================
  Coverage   32.65%   32.65%           
=======================================
  Files          87       87           
  Lines       26158    26158           
  Branches     6874     6874           
=======================================
  Hits         8541     8541           
+ Misses      16656    16645   -11     
- Partials      961      972   +11

Impacted Files	Coverage Δ
rmgpy/data/statmech.py	42.39% <0%> (ø)	⬆️
rmgpy/rmg/pdep.py	12.21% <0%> (ø)	⬆️
rmgpy/reaction.py	0% <0%> (ø)	⬆️
rmgpy/data/kinetics/family.py	48.49% <0%> (ø)	⬆️
rmgpy/yml.py	15.71% <0%> (ø)	⬆️
rmgpy/data/kinetics/database.py	50.73% <0%> (ø)	⬆️
rmgpy/rmg/input.py	34.34% <0%> (ø)	⬆️
rmgpy/molecule/molecule.py	0% <0%> (ø)	⬆️
rmgpy/statmech/ndTorsions.py	59.89% <0%> (ø)	⬆️

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[amarkpayne]

@ajocher if this looks good to you then you can merge this in as soon as the tests pass.

[amarkpayne]

@mliu49 I have also cherry-picked and pushed this to the v2.5.0 branch as it should be included in our legacy python 2 branch