fix: correct indentation for Install Apptainer step in main.yml

Author: lewisl23

.github/workflows/main.yml

@@ -47,7 +47,7 @@ jobs:
           pip install snakemake
           pip install pulp==2.7.0
 
-        - name: Install Apptainer
+      - name: Install Apptainer
         run: |
           sudo apt-get update
           sudo apt-get install -y apptainer