Fix typos discovered by codespell

[cclauss]

Fix typos discovered by codespell - https://pypi.org/project/codespell

% codespell --ignore-words-list=canbe,hel,te

./h11/_headers.py:47: occurences ==> occurrences
./h11/_headers.py:84: comparisions ==> comparisons
./h11/tests/test_connection.py:655: re-use ==> reuse
./h11/tests/test_state.py:242: Succesful ==> Successful
./docs/source/api.rst:205: re-use ==> reuse
./docs/source/api.rst:284: unparseable ==> unparsable
./docs/source/api.rst:474: re-used ==> reused
./docs/source/api.rst:510: re-use ==> reuse
./docs/source/api.rst:521: re-use ==> reuse
./docs/source/api.rst:608: Re-using ==> Reusing
./docs/source/api.rst:611: re-used ==> reused
./docs/source/api.rst:638: re-use ==> reuse
./docs/source/api.rst:654: re-use ==> reuse
./docs/source/basic-usage.rst:289: re-use ==> reuse
./docs/source/changes.rst:296: verson ==> version
./examples/trio-server.py:44: debateable ==> debatable
./examples/trio-server.py:57: debateable ==> debatable
./examples/trio-server.py:265: re-use ==> reuse

% codespell --ignore-words-list=canbe,hel,te --write-changes