doc: mention markdown linting in BUILDING.md

BUILDING.md mentioned JS and C++ linting were performed
in the 'make lint' command, but 'make lint' also
performs markdown linting. This commit updates the docs
to include markdown as one of the things being linted.

BUILDING.md

@@ -322,8 +322,7 @@ Make sure the linter does not report any issues and that all tests pass. Please
 do not submit patches that fail either check.
 
 If you want to run the linter without running tests, use
-`make lint`/`vcbuild lint`. It will run both JavaScript linting and
-C++ linting.
+`make lint`/`vcbuild lint`. It will lint JavaScript, C++, and Markdown files.
 
 If you are updating tests and want to run tests in a single test file
 (e.g. `test/parallel/test-stream2-transform.js`):