Improper bus format for GHA gas pipeline

Checklist

[ ] I am using the current main branch or the latest release. Please indicate.
[ ] I am running on an up-to-date pypsa-africa environment. Update via conda env update -f envs/environment.yaml.

For some reason, while building the model for Africa, a gas pipeline has as bus "GHA15_2" that should instead be "GHA.15_2" or alike