Correct the directory in the layout-test build instruction

Change-Id: I80c5bea68c43493acb0b6cb95b9263adf51f856a
Reviewed-on: https://chromium-review.googlesource.com/c/1351209
Commit-Queue: Robert Ma <robertma@chromium.org>
Reviewed-by: Robert Ma <robertma@chromium.org>
Cr-Commit-Position: refs/heads/master@{#610859}

docs/testing/layout_tests.md

@@ -23,7 +23,7 @@ Before you can run the layout tests, you need to build the `blink_tests` target
 to get `content_shell` and all of the other needed binaries.
 
 ```bash
-autoninja -C out/Release blink_tests
+autoninja -C out/Default blink_tests
 ```
 
 On **Android** (layout test support