Add one more example to "git try -h" output.

A lot of folks have asked me how to do this while reviewing chrome
changes with dependant blink changes, or when they're starting blink
development. Hopefully this extra example makes figuring this out
easier for blink developers.

R=dpranke@chromium.org
BUG=None

Review URL: https://codereview.chromium.org/97983004

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/depot_tools@243290 0039d316-1c4b-4281-b951-d872f2087c98

trychange.py

@@ -52,6 +52,9 @@ Examples:
   Try a change against a particular revision:
     %(prog)s -r 123
 
+  Try a change including changes to a sub repository:
+    %(prog)s -s third_party/WebKit
+
   A git patch off a web site (git inserts a/ and b/) and fix the base dir:
     %(prog)s --url http://url/to/patch.diff --patchlevel 1 --root src