doc: Add --merge-mode-functions=separate option to gcovr invocations

Otherwise, commands fail with the error:
```
Stderr of gcov was >><< End of stderr
Exception was >>Got function secp256k1_scalar_split_lambda on multiple lines: 67, 142.
	You can run gcovr with --merge-mode-functions=MERGE_MODE.
	The available values for MERGE_MODE are described in the documentation.<< End of stderr
```

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*' --exclude 'src/modules/.*/bench_impl.h' --print-summary
+    $ gcovr --merge-mode-functions=separate --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*' --exclude 'src/modules/.*/bench_impl.h' --html --html-details -o coverage/coverage.html
+    $ gcovr --merge-mode-functions=separate --exclude 'src/bench*' --exclude 'src/modules/.*/bench_impl.h' --html --html-details -o coverage/coverage.html
 
 #### Exhaustive tests