All text seems very plain and normal it doesn't stand out as an update at all and I can see someone simply closing it or moving to another page without realising an update has taken place.
One step would be to bold the text:
"Tor Browser has been updated."
"Changelog:"
Secondly may not be related, but on tor browser startup the url bar is empty and it doesn't say whether it is about:tor or about:tbupdate why is this?