Fixed broken link in ActivityInstrumentation sample description.

Change-Id: I1ab9a5750693f44dec7aa97c99a2cfcd6af0b78e

Author: qchong

samples/browseable/ActivityInstrumentation/_index.jd

@@ -9,5 +9,5 @@ sample.group=Testing
 {@link android.test.InstrumentationTestCase} to test the internal state of an
 {@link android.app.Activity}.</p>
 <p>To learn more about using Android's custom testing framework, see
-<a href="{@docRoot}/training/activity-testing/index.html">Testing Your
+<a href="{@docRoot}training/activity-testing/index.html">Testing Your
 Android Activity</a>.</p>