Fix for code sample in Time.zone= comment

Hi,

This is just a small update to the docs for Time.zone=.

Even though Time.zone is thread-local, it will leak across requests within the same thread if you handle one request with a logged-in user and the next request with a logged-out user.

This little change updates the sample code to use an around_filter instead of a before_filter to show how to fix this.

P.S. You might be tempted to combine the first two lines of set_time_zone into one line with the comma assignment syntax and add "if logged_in? to the one line in the ensure clause. Don't do it. If you did that, the user's time zone would leak out to the next request when the user logs out.