Use major version for version expression in the example in README (#52)

This is a follow-up to #51.

README.md

@@ -35,7 +35,7 @@ jobs:
         with:
           python-version: ${{ matrix.python-version }}
       - name: Run image
-        uses: abatilo/actions-poetry@v2.0.0
+        uses: abatilo/actions-poetry@v2
         with:
           poetry-version: ${{ matrix.poetry-version }}
       - name: View poetry --help