Correct skip slow specification in workflow

Author: matt-graham

.github/workflows/tests.yml

@@ -57,7 +57,7 @@ jobs:
     - name: Run tests (skipping slow)
       if: github.event_name == 'pull_request'
       run: |
-        pytest -v --cov-report=xml --cov=s2fft --cov-config=.coveragerc -m "no slow"
+        pytest -v --cov-report=xml --cov=s2fft --cov-config=.coveragerc -m "not slow"
 
     - name: Run tests
       if: github.event_name != 'pull_request'