docs: Exclude modules' bench_impl.h headers from coverage report

Author: hebasto

CONTRIBUTING.md

@@ -92,12 +92,12 @@ Run the tests:
 
 To create a report, `gcovr` is recommended, as it includes branch coverage reporting:
 
-    $ gcovr --exclude 'src/bench*' --print-summary
+    $ gcovr --exclude 'src/bench*' --exclude 'src/modules/.*/bench_impl.h' --print-summary
 
 To create a HTML report with coloured and annotated source code:
 
     $ mkdir -p coverage
-    $ gcovr --exclude 'src/bench*' --html --html-details -o coverage/coverage.html
+    $ gcovr --exclude 'src/bench*' --exclude 'src/modules/.*/bench_impl.h' --html --html-details -o coverage/coverage.html
 
 #### Exhaustive tests