Update instructions to trigger try builds for forks (#47)

Signed-off-by: webbeef <me@webbeef.org>

src/hacking/testing.md

@@ -89,7 +89,7 @@ Instead, consider building with `mach build -r` and testing with `mach test-wpt
 ### Running tests on your GitHub fork
 
 Alternatively, you can execute the tests on GitHub-hosted runners using `mach try`.
-Usually, `mach try wpt-2020` (all tests, linux, `layout-2020`) will be enough.
+Usually, `mach try linux-wpt-2020` (all tests, linux, `layout-2020`) will be enough.
 
 You can view the run results in your fork under the "Actions" tab.
 Any failed tasks will include a list of stable unexpected results at the bottom of the log.