Fix documentation for /api/v4/projects/:id/repository/archive

The documentation mistakenly says that `format` can be used as a
parameter, but the Grape middleware reserves `format` as a content-type
header. Update the documentation to make it explicit the suffix
should be used to specify a format.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/45992

doc/api/repositories.md

@@ -108,14 +108,18 @@ Get an archive of the repository. This endpoint can be accessed without
 authentication if the repository is publicly accessible.
 
 ```
-GET /projects/:id/repository/archive
+GET /projects/:id/repository/archive[.format]
 ```
 
+`format` is an optional suffix for the archive format. Default is
+`tar.gz`. Options are `tar.gz`, `tar.bz2`, `tbz`, 'tbz2`, `tb2`,
+`bz2`, `tar`, and `zip`. For example, specifying `archive.zip`
+would send an archive in ZIP format.
+
 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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