Refer to Zulip channel with its new name #t-clippy (#17610)

Fixes problem [reported in
Zulip](https://rust-lang.zulipchat.com/#narrow/channel/257328-t-clippy/topic/The.20clippy.20channel.20link.20for.20zulip.20doesn.27t.20seem.20to.20work/with/618159412).

r? @Manishearth

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