[WebSocket] Fix a handshake failure message.

Fix a grammar error on a handshake failure message.

BUG=NONE
R=ricea@chromium.org

Review URL: https://codereview.chromium.org/107923002

git-svn-id: svn://svn.chromium.org/blink/trunk@163342 bbb929c8-8fbe-4397-9dbb-9b2b20218538