emacs: fix GitLab API usage when generating MELPA packages

The wrong API fetches a tarball from master instead of the specified
commit, leading to a wrong hash[1].

API: https://docs.gitlab.com/api/repositories/#get-file-archive

[1]: https://github.com/nix-community/emacs-overlay/pulls?q=is%3Apr+author%3Ajian-lin+is%3Aclosed+in%3Atitle+hash+

pkgs/applications/editors/emacs/elisp-packages/update-melpa.el

@@ -101,7 +101,7 @@ return Promise to resolve in that process."
             ("gitlab"    (list "nix-prefetch-url"
                                "--unpack" (concat "https://gitlab.com/api/v4/projects/"
                                                   (url-hexify-string repo)
-                                                  "/repository/archive.tar.gz?ref="
+                                                  "/repository/archive.tar.gz?sha="
                                                   commit)))
             ("sourcehut" (list "nix-prefetch-url"
                                "--unpack" (concat "https://git.sr.ht/~" repo "/archive/" commit ".tar.gz")))