Update test.md doc to fix sample test regex
Remove the extra dollar sign from the test regex so the command
would find the desired test.
@@ -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='github.com/docker/docker/opts' TESTFLAGS='-test.run $^TestValidateIPAddress$' make test-unit
+$ TESTDIRS='github.com/docker/docker/opts' TESTFLAGS='-test.run ^TestValidateIPAddress$' make test-unit
```
## Run integration tests
