update coverage path

Author: simonrw

.github/workflows/coverage.yml

@@ -32,7 +32,7 @@ jobs:
       - name: Run cargo-tarpaulin
         uses: actions-rs/tarpaulin@v0.1
         with:
-          args: --manifest-path fitsio --coveralls ${{ secrets.COVERALLS_TOKEN }}
+          args: --manifest-path fitsio/Cargo.toml --coveralls ${{ secrets.COVERALLS_TOKEN }}
 
       - name: Archive code coverage results
         uses: actions/upload-artifact@v1