Document format parameter for Repository Archive endpoint

doc/api/repositories.md

@@ -114,6 +114,7 @@ Parameters:
 - `id` (required) - The ID or [URL-encoded path of the project](README.md#namespaced-path-encoding) owned by the authenticated user
 - `sha` (optional) - The commit SHA to download. A tag, branch reference or sha can be used. This defaults to the tip of the default branch if not specified
+- `format` (optional) - The archive format. Default is `tar.gz`. Options are `tar.gz`, `tar.bz2`, `tbz`, `tbz2`, `tb2`, `bz2`, `tar`, `zip`
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