Fix update mirror tooltip.

app/views/projects/buttons/_update_mirror.html.haml

 - if @project.mirror? && can?(current_user, :push_code, @project)
   - size = nil unless defined?(size) && size
   - if @project.updating_mirror?
-    %span.btn.disabled.update-mirror-button.has_tooltip{title: "Updating from mirror..."}
+    %span.btn.disabled.update-mirror-button.has_tooltip{title: "Updating from upstream..."}
       = icon('refresh')
   - else
-    = link_to update_now_namespace_project_mirror_path(@project.namespace, @project), method: :post, class: "btn update-mirror-button has_tooltip", title: "Update from mirror" do
+    = link_to update_now_namespace_project_mirror_path(@project.namespace, @project), method: :post, class: "btn update-mirror-button has_tooltip", title: "Update from upstream" do
       = icon('refresh')