spec: fix spelling error in docstring for fs.walk

[skshetry]

No description provided.

[skshetry]

@martindurant, would you be open to add a pre-commit hook for codespell instead of using github actions? pre-commit hooks are pinned to a specific version, so they are less likely to fail unexpectedly.

(I might be overlooking reasons for using a GHA, though.)

[martindurant]

would you be open to add a pre-commit hook for codespell instead of using github actions?

Sure. This was probably meant to be there already (and my IDE does it automatically for me anyway, which is why I wouldn't have noticed).

[martindurant]

Merging this - remaining CI issues can be fixed elsewhere.