Fix description of Page.getCookies in protocol.json.

Page.getCookies has 'cookies' field, not cookie.
So this CL makes it correct.

BUG=none.

Review URL: https://codereview.chromium.org/107433002

git-svn-id: svn://svn.chromium.org/blink/trunk@163334 bbb929c8-8fbe-4397-9dbb-9b2b20218538