Fix label for landice mesh_modifications test group in dev guide

[matthewhoffman]

This PR fixes the label for the landice mesh_modifications test group in the devevlopers guide so it does not conflict with the section in the users guide with the same name.

Checklist

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

[matthewhoffman]

Testing

I compiled the docs and ensured that the associated warning is now gone.

[xylar]

Thanks, @matthewhoffman! I lost track of this one.