README: Fix the Yahoo example

The `browser.title` only contains 'Yahoo' now (without the '!'):

>>> from selenium import webdriver
>>> browser = webdriver.Firefox()
>>> browser.get('http://www.yahoo.com')
>>> browser.title
u'Yahoo'

So the assert statement fails. This commit fixes the problem.

Signed-off-by: Andreas Tolfsen <ato@mozilla.com>