Missing comma in `search_fields` breaks /admin search

[code-review-doctor]

search_fields should be a list or tuple according to django docs but it's a str:

openstax-cms/webinars/wagtail_hooks.py

Line 10 in ad9cd28

 search_fields = ('title') 

Notice the lack of a comma, so this gets evaluated to a str. A one item tuple must end in a comma so Python knows it's a tuple.

I can make a PR to fix this for you?