minor fix to auxvargrad in axisymmetric source residual

[jtneedels]

Signed-off-by: jtneedels jneedels@stanford.edu

Proposed Changes

Correction of indexing in NEMO axisymmetric source residual, consistent with change in perfect gas source residual in develop.

Related Work

Related to #1192

PR Checklist

Put an X by all that apply. You can fill this out after submitting the PR. If you have any questions, don't hesitate to ask! We want to help. These are a guide for you to know what the reviewers will be looking for in your contribution.

• I am submitting my contribution to the develop branch.
• My contribution generates no new compiler warnings (try with the '-Wall -Wextra -Wno-unused-parameter -Wno-empty-body' compiler flags, or simply --warnlevel=2 when using meson).
• My contribution is commented and consistent with SU2 style.
• I have added a test case that demonstrates my contribution, if necessary.
• I have updated appropriate documentation (Tutorials, Docs Page, config_template.cpp) , if necessary.