Fix typos discovered by codespell

[cclauss]

% codespell # https://pypi.org/project/codespell

./tests/data/swagger_openapi_v3.0.1.json:210: Muliple ==> Multiple
./openapi_pydantic/v3/v3_0/README.md:29: constriants ==> constraints
./openapi_pydantic/v3/v3_0/schema.py:135: preferrably ==> preferably
./openapi_pydantic/v3/v3_1/README.md:31: constriants ==> constraints

% codespell --write-changes

[mike-oakley]

Thanks!