Wrong link in chat documentation

In https://tails.boum.org/doc/anonymous_internet/pidgin, section ‘IRC servers blocking Tor’, we’re linking to freenode.net doc :

https://freenode.net/irc_servers.shtml#tor

however, this page doesn’t exist anymore, we should use this link instead :

https://freenode.net/kb/answer/chat

Original created by @goupille on 16551 (Redmine)

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