Fix name of configure_polaris_envs.py in docs

[xylar]

Checklist

• [x] Developer's Guide has been updated
• [x] Documentation has been built locally and changes look as expected

closes #30

[xylar]

@alicebarthel, thanks for reporting this! It should be fixed here. Let me know if you notice any instances I missed.

[xylar]

BTW, I wanted to clarify that multiple environments (conda and spack) are being configured. I guess I didn't always remember!

[xylar]

@alicebarthel, a visual inspection is perfectly adequate here.

[xylar]

Thanks very much!