Fix typo in adding_lints.md

[dp304]

changelog: none

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @llogiq (or someone else) soon.

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[llogiq]

Thank you! @bors r+

[bors]

📌 Commit 22a958d has been approved by llogiq

[bors]

⌛ Testing commit 22a958d with merge fbc1eeb...

[bors]

💔 Test failed - checks-action_test

[llogiq]

Error seems unrelated, we may need a rustup here.

[llogiq]

Let's rebase when #6277 lands.

[dp304]

Rebased.
I have a probably unrelated question: rustfmt is unavailable in the nightly builds since 2020-10-25, but that build is too old, and seems to conflict with new changes in clippy.
How can I set up the dev environment so that the most recent rustc-nightly is available as well as a recent enough rustfmt?

[llogiq]

Type ./setup_environment.sh in your clippy project folder.

@bors r+

[bors]

📌 Commit edfa13d has been approved by llogiq

[bors]

⌛ Testing commit edfa13d with merge e298c83...

[bors]

☀️ Test successful - checks-action_dev_test, checks-action_remark_test, checks-action_test
Approved by: llogiq
Pushing e298c83 to master...