Update exclude_path description and examples

Now that we use cargo-llvm-cov, `exclude_path` should be a regular
expression, since we are using `--ignore-filename-regex` option:

   --ignore-filename-regex <PATTERN>
       Skip source code files with file paths that match the given
       regular expression

To prevent users from separating multiple files with commas,
let's update the `exclude_path` description in the README and in
the examples.

Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>

coverage_config_x86_64.json.sample

@@ -1,5 +1,5 @@
 {
   "coverage_score": 90,
-  "exclude_path": "my_autogenerated_file.rs,second_file.rs",
+  "exclude_path": "my_autogenerated_file\\.rs|path/to/my_folder",
   "crate_features": "my_dummy_feature,dummy_feature2"
 }