nix-prefetch-git: list --branch-name in help

Listing the option and brief explanation.
Addresses: https://github.com/NixOS/nixpkgs/issues/74182

pkgs/build-support/fetchgit/nix-prefetch-git

@@ -42,6 +42,7 @@ Options:
       --url url       Any url understood by 'git clone'.
       --rev ref       Any sha1 or references (such as refs/heads/master)
       --hash h        Expected hash.
+      --branch-name   Branch name to check out into
       --deepClone     Clone the entire repository.
       --no-deepClone  Make a shallow clone of just the required ref.
       --leave-dotGit  Keep the .git directories.