Fix mistake in docs/contributing/test.md running specific unit test (#38096)

Signed-off-by: Jonathan A. Schweder <jonathanschweder@gmail.com>

docs/contributing/test.md

@@ -148,7 +148,7 @@ You can use the `TESTDIRS` environment variable to run unit tests for
 a single package.
 
 ```bash
-$ TESTDIRS='opts' make test-unit
+$ TESTDIRS='github.com/docker/docker/opts' make test-unit
 ```
 
 You can also use the `TESTFLAGS` environment variable to run a single test. The
@@ -163,7 +163,7 @@ On unit tests, it's better to use `TESTFLAGS` in combination with
 `TESTDIRS` to make it quicker to run a specific test.
 
 ```bash
-$ TESTDIRS='opts' TESTFLAGS='-test.run ^TestValidateIPAddress$' make test-unit
+$ TESTDIRS='github.com/docker/docker/opts' TESTFLAGS='-test.run $^TestValidateIPAddress$' make test-unit
 ```
 
 ## Run integration tests