modifying correctly the workflow

.github/workflows/python-package.yml

@@ -29,7 +29,7 @@ jobs:
         python -m pip install --upgrade pip
         python -m pip install flake8 pytest
         if [ -f requirements.txt ]; then pip install -r requirements.txt; fi
-        pip install py21cmcast
+        python -m pip install py21cmcast
     - name: Lint with flake8
       run: |
         # stop the build if there are Python syntax errors or undefined names