Fix variable name in FluxDiagram script

[hwpang]

Description of Changes

Fix the naming bug for argument parsing

[codecov]

Codecov Report

Merging #1759 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #1759   +/-   ##
======================================
  Coverage    32.6%   32.6%           
======================================
  Files          87      87           
  Lines       26115   26115           
  Branches     6875    6875           
======================================
  Hits         8516    8516           
+ Misses      16641   16629   -12     
- Partials      958     970   +12

Impacted Files	Coverage Δ
rmgpy/data/statmech.py	42.2% <0%> (ø)	⬆️
rmgpy/rmg/pdep.py	12.21% <0%> (ø)	⬆️
rmgpy/reaction.py	0% <0%> (ø)	⬆️
rmgpy/data/kinetics/family.py	48.28% <0%> (ø)	⬆️
rmgpy/yml.py	15.71% <0%> (ø)	⬆️
rmgpy/data/kinetics/database.py	50.61% <0%> (ø)	⬆️
rmgpy/molecule/molecule.py	0% <0%> (ø)	⬆️
rmgpy/statmech/ndTorsions.py	59.78% <0%> (ø)	⬆️

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[mliu49]

Thanks!