Fix matrix value for non-default py version example (#100)

README.md

@@ -87,7 +87,7 @@ jobs:
     - uses: actions/checkout@v2
     - uses: actions/setup-python@v2
       with:
-        python-version: ${{ matrix.python }}
+        python-version: ${{ matrix.python-version }}
     - run: python my_script.py
 
 ```