Changing submodule names to reflect new repository names

[fmalatino]

Description
This PR changes the .gitmodules file to match the new names for the pyFV3 and pySHiELD submodules. Previously the container build test was failing due to the conflict in names.

How Has This Been Tested?
Tested using currently implemented workflows.

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• Any dependent changes have been merged and published in downstream modules
• New check tests, if applicable, are included

[jjuyeonkim]

Besides the pyFV3 linting, it looks okay to me.