doc: improve example single-test command

Changes example command to refer to the full test directory:
`./test/parallel/` instead of `./parallel/`

PR-URL: nodejs#29171
Reviewed-By: Rich Trott <rtrott@gmail.com>
Reviewed-By: Trivikram Kamat <trivikr.dev@gmail.com>

BUILDING.md

@@ -328,7 +328,7 @@ If you are updating tests and want to run tests in a single test file
 (e.g. `test/parallel/test-stream2-transform.js`):
 
 ```text
-$ python tools/test.py parallel/test-stream2-transform.js
+$ python tools/test.py test/parallel/test-stream2-transform.js
 ```
 
 You can execute the entire suite of tests for a given subsystem