[BUG] Title information missing from some lecture navigation buttons

[mmcky]

trafficstars

http://preview.python-advanced.quantecon.org.s3-website-ap-southeast-2.amazonaws.com/muth_kalman.html

doesn;'t have titles for stationary_densities as the next lecture.

Rating this as minor as the buttons look ok and are functional. But it would be good to fix in the generated notebook metadata