Link to the `channel` documentation in `sync` page is broken

[cagatay-y]

Documentation page for the sync module links to the channel function which was removed in the last version.
I originally intended to open a PR with a fix but I am not sure whether it would be more appropriate to change the link to the new channel module, or remove the link altogether since the functionality is moved from sync.

[yoshuawuyts]

@cagatay-y thanks for opening this! -- We probably should remove the existing mention of channels, and instead mention something like: "if you're looking for channels, check out async_std::channel. Would you be willing to open a PR for that?