Update trac links in extension description

The descriptions of the Firefox and Chrome extensions have a link pointing to trac.torproject.org:

For more detailed information about how Snowflake works and for instructions on how to report bugs, see our documentation wiki at https://trac.torproject.org/projects/tor/wiki/doc/Snowflake/. For more information about Tor Project, visit https://www.torproject.org/.

I suggest this should point to https://snowflake.torproject.org/ instead.

The trac link is bad because it redirects to a "Legacy" part of the GitLab wiki, https://gitlab.torproject.org/legacy/trac/-/wikis/doc/Snowflake/, and it has broken markup which I don't have permissions to fix. (The up-to-date wiki page is https://gitlab.torproject.org/tpo/anti-censorship/pluggable-transports/snowflake/-/wikis/home.)

I'm actually not sure where the description text is stored. It's not in package.json. It might be something that someone has to log in to the add-ons store pages to fix.