Fix icon reference in project menu

Author: oliverguenther

lib/open_project/proto_plugin/engine.rb

@@ -43,7 +43,7 @@ class Engine < ::Rails::Engine
            after: :overview,
            param: :project_id,
            caption: "Kittens",
-           icon: 'icon2 icon-settings',
+           icon: :squirrel,
            html: { id: "kittens-menu-item" },
            if: ->(project) { true }