announce is a list rather than a string

[Marcin Kurczewski]

The announce URL is serialized as a list containing a single element.
I think it should be serialized as a string since there's announce-list.

Sources:

• http://www.bittorrent.org/beps/bep_0003.html
• http://www.bittorrent.org/beps/bep_0012.html