📚 Fix typos discovered by codespell

[cclauss]

https://pypi.org/project/codespell

% codespell --ignore-words-list=nd,rsource --quiet-level=3

./CHANGELOG.md:171: suppressable ==> suppressible
./CHANGELOG.md:172: suppressable ==> suppressible
./docs/develop/background.md:25: converstaion ==> conversation
./docs/syntax/cross-referencing.md:17: bloc ==> block
./docs/syntax/math.md:74: multline ==> multiline
./docs/syntax/optional.md:221: multline ==> multiline
./myst_parser/warnings_.py:58: occured ==> occurred
./myst_parser/parsers/directives.py:272: convertor ==> converter
./myst_parser/parsers/directives.py:279: convertor ==> converter
./myst_parser/parsers/directives.py:284: convertor ==> converter
./myst_parser/mdit_to_docutils/sphinx_.py:185: referencable ==> referenceable

% codespell --ignore-words-list=nd,rsource --quiet-level=3 --write-changes

[chrisjsewell]

cheers!

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.30%. Comparing base (8ea5645) to head (5df8fa9).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #969   +/-   ##
=======================================
  Coverage   90.30%   90.30%           
=======================================
  Files          24       24           
  Lines        3507     3507           
=======================================
  Hits         3167     3167           
  Misses        340      340           

Flag	Coverage Δ
pytests	90.30% <100.00%> (ø)

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