Update wagtail imports

[danbentley]

I've updated the documentation to match the module path changes made in Wagtail 2.0. https://github.com/wagtail/wagtail/blob/882f8f3cf8ddd79c30e611a48882b309e90dad0c/docs/releases/2.0.rst#wagtail-module-path-updates

I left the reference to wagtailcore in the 2.1.2 release notes since the module path was accurate at the time though happy to update that too.

https://github.com/rkhleics/wagtailmenus/blob/master/docs/source/releases/2.1.2.rst

Additionally, I've fixed an issue in the hook example code. Happy to break out into a separate PR if you prefer.

Thanks!

[codecov[bot]]

Codecov Report

Merging #379 into master will increase coverage by 0.19%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #379      +/-   ##
==========================================
+ Coverage   98.07%   98.26%   +0.19%     
==========================================
  Files          88       89       +1     
  Lines        2855     2945      +90     
==========================================
+ Hits         2800     2894      +94     
+ Misses         55       51       -4     

Impacted Files	Coverage Δ
wagtailmenus/errors.py	100.00% <0.00%> (ø)
wagtailmenus/utils/deprecation.py	0.00% <0.00%> (ø)
wagtailmenus/utils/tests/test_misc.py	100.00% <0.00%> (ø)
wagtailmenus/tests/test_base_menu_classes.py	100.00% <0.00%> (ø)
wagtailmenus/models/menus.py	97.97% <0.00%> (+0.06%)	:arrow_up:
wagtailmenus/utils/misc.py	94.36% <0.00%> (+1.38%)	:arrow_up:
wagtailmenus/utils/version.py	100.00% <0.00%> (+15.38%)	:arrow_up:

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