Fixed the tweet_button example

The following tweet_button example will give Syntax error
<%= tweet_button(via: => "peterc", url: "http://news.ycombinator.com", :text => "AWESOME.") %> 

The issue is with following
via: => "peterc" 
The hash is not defined correctly. It should be either 
via: "peterc" 
or 
:via => "peterc"