docs: Fix a few typos

[timgates42]

There are small typos in:

• docs/source/serializers.rst
• tests/unit/test_serializers.py

Fixes:

• Should read serialize rather than seralize.
• Should read behaviour rather than beaviour.

Semi-automated pull request generated by https://github.com/timgates42/meticulous/blob/master/docs/NOTE.md

[hroncok]

Thanks.