documentation - extending generic views - fix typo
documentation - extending generic views - fix typo

- closing ``` (backticks) were missing

rsalmaso/wagtail,thenewguy/wagtail,zerolab/wagtail,wagtail/wagtail,rsalmaso/wagtail,zerolab/wagtail,wagtail/wagtail,zerolab/wagtail,rsalmaso/wagtail,thenewguy/wagtail,zerolab/wagtail,wagtail/wagtail,rsalmaso/wagtail,thenewguy/wagtail,zerolab/wagtail,rsalmaso/wagtail,wagtail/wagtail,thenewguy/wagtail,wagtail/wagtail,thenewguy/wagtail