Fix `last_activity_at` to `last_activity_on`

Signed-off-by: Rémy Coutable <remy@rymai.me>

spec/requests/git_http_spec.rb

@@ -160,7 +160,7 @@ describe 'Git HTTP requests', lib: true do
 
             it 'updates the user last activity' do
               download(path, {}) do |response|
-                expect(user.reload.last_activity_at).not_to be_nil
+                expect(user.reload.last_activity_on).not_to be_nil
               end
             end
           end