fixed bug reported on homepage

Hi! 
From the homepage at http://www.dalkescientific.com/Python/PyRSS2Gen.html :
" Line 353 has _opt_element(handler, "tt", ttl) but should be _opt_element(handler, "ttl", ttl) "

Just thought I'd be polite and pass on the fix -- apparently there is no canonical repo for this library, so yours might end up being the de-facto one.

Cheers
Giacomo