message: docs: fix a load-bearing typo; add a link to gcovr
diff: @@ -214,8 +214,9 @@ https://github.com/bazelbuild/bazel/issues/2805.
  # Coverage builds
  -To generate coverage results, make sure you have `gcov` 3.3 in your `PATH` (or
  -set `GCOVR` to point at it). Then run:
  +To generate coverage results, make sure you have
  +[`gcovr`](https://github.com/gcovr/gcovr) 3.3 in your `PATH` (or set `GCOVR` to
  +point at it). Then run:
  ```
  test/run_envoy_bazel_coverage.sh
  