This attribute is called caption  now, not title , to be consistent across all resource types inserted as figures:

https://leanpub.com/markua/read#leanpub-auto-figures
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So, you'll want to do a query-replace of title:  to caption: 

I recently finished the first complete draft of the Markua spec https://leanpub.com/markua/read including support for courses, and Scott recently made this change to code samples.  I should have posted specifically about the title  to caption  change; sorry.

Finally, we now have a reasonably complete list of what is still TODO in Leanpub's Markua support here:

https://leanpub.com/markua/read#leanpub-auto-todo-in-leanpub
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