[Theme: Twenty Thirteen] Remove "Comment on " hover text!

When you are hovering with your mouse over the “Leave a comment”-text below a post using the “Thirty Thirteen” theme, you get a text similar to the “alt=” text on an image. The text reads “Comment on <post title>”.

How do I remove this? I have been into the code and changed the “Leave a comment”-text to something else, but I could not find the words “Comment on” anywhere. In fact, I have clicked and searched through every single file in the sidebar of the “Edit” section of “Appearance” in WP, and I can’t find the words anywhere.