Change ghcr branch name to develop in README

Author: cailmdaley

README.md

@@ -42,7 +42,7 @@ We recommend running the image with **Apptainer** (formerly Singularity) which i
 ```bash
 # build writeable "sandbox" container in the current directory
 # ./sp_validation will be a directory that functions like a vm
-apptainer build --sandbox sp_validation docker://ghcr.io/cosmostat/sp_validation:pyproject_docker
+apptainer build --sandbox sp_validation docker://ghcr.io/cosmostat/sp_validation:develop
 
 # open a shell in the container
 apptainer shell --writable sp_validation