Docs: Fix manifest example for ARC

Simply add "android:" qualifier to required attribute.

Bug: 31554344
Test: None

Change-Id: I13ef9c8614d76bd99f9d7b69d3ea8319843e979a

docs/html/topic/arc/index.jd

@@ -51,7 +51,7 @@ review your mouse and keyboard interactions.
     <!-- Some Chromebooks don't support touch. Although not essential,
          it's a good idea to explicitly include this declaration. -->
     <uses-feature android:name="android.hardware.touchscreen"
-                  required="false" />
+                  android:required="false" />
 </manifest>
 </pre>