extensions/shell: Update README file with the recent flag change.

The --app flag is gone, now it is called --app-shel-app-path.

BUG=None
TEST=see README
R=jamescook@chromium.org
TBR=yoz@chromium.org
NOTRY=true

Review URL: https://codereview.chromium.org/414223010

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@285862 0039d316-1c4b-4281-b951-d872f2087c98

extensions/shell/README

@@ -10,4 +10,8 @@ app_shell is only officially supported on Chrome OS.
 
 # How to run
 
-app_shell --app=/path/to/extension
+$ app_shell --app-shell-app-path=/path/to/extension
+
+# For example, you can try the calculator app:
+
+$ app_shell --app-shell-app-path=chrome/common/extensions/docs/examples/apps/calculator/app/