doc: update wrongly formatted alert in nixpkgs README.md

pkgs/README.md

···
 It is recommended to keep the PR title in sync with the commit title, to make it easier to find.
 For PRs with multiple commits, the PR title should be a general summary of these commits.
 
-[!NOTE]
-Marking a PR as a draft does not prevent automatic builds.
+> [!NOTE]
+> Marking a PR as a draft does not prevent automatic builds.
 
 ## Category Hierarchy
 [categories]: #category-hierarchy