Improve-Coverage.md

@@ -3,7 +3,9 @@ To obtain the detailed code coverage report, you can either
 - locally, run 
    ```
    cmake .. -DCMAKE_BUILD_TYPE=Coverage -DUSE_Pythia8_C8=C8
-   make -j4 && ctest -j4 && make coverage
+   make -j4 && ctest -j4 
+   rm -rf raw-coverage.info coverage*
+   make coverage
    ```
 
 In both cases, the coverage is accessible in `coverage-report/index.html`.