revert the Discord link change from c1f18dd

actually there are link *variables* at the bottom of the document
which makes it easier to read i think and the link to the `#nufmt`
channel is wrong:
- https://discord.com/channels/601130461678272522/1117921521520873623 is
  the new `#nufmt` channel
- https://discord.com/channels/601130461678272522/1087437882396004442
  was the link of the closed `#nufmt` threads under the `#general`
  channel

docs/CONTRIBUTING.md

@@ -4,7 +4,7 @@ Salutations! Thanks for coming by and the interest into this project!
 We would like to order the contributions like this:
 
 - Before you start hacking, it is important to **ask first** if your idea or bugfix is in order.
- Create an issue or come and say hi in the [`#nufmt` channel](https://discord.com/channels/601130461678272522/1087437882396004442) by joining [the discord](https://discord.gg/NtAbbGn).
+ Create an issue or come and say hi in the [`#nufmt` channel][nufmt discord channel] by joining [the discord][Nushell discord].
  We don't bite!.
 
  It would be sad that you do the effort to clone the project, successfully make the PR, but it wasn't in our plans or there is another PR that is currently adressing that issue.