DevTools: update protocol description for CSS.setPropertyText method

This patch updates description for the CSS.setPropertyText method to
reflect changes in r170862

Review URL: https://codereview.chromium.org/227533003

git-svn-id: svn://svn.chromium.org/blink/trunk@170973 bbb929c8-8fbe-4397-9dbb-9b2b20218538