Use "committish" rather than "commitish" in docs

[philrz]

What's Changing

This proposes using the Git-aligned spelling "committish" rather than the "commitish" that currently appears in the docs.

The "commitish" spelling also appears many places in the code, so if we make the change at the docs layer we might want to think about doing it in the code to match.

Why

The term originates with Git, whose glossary defines it as "commit-ish (also committish)". Both of the spellings Git sanctions have either the hyphen or the double "t". Ours has neither.

Git's own practice is lopsided. Counts of each spelling inside the git/git repository, via GitHub code search:

Spelling	Occurrences in git/git
committish	40
commit-ish	33
commitish	1

Note: That single "commitish" hit in git/git is related to the change 84a7c33a4b that was their own similar spelling adjustment. Its message reads "The git glossary accepts a hyphen or a double-t, but not a single-t" and "none of the typos appear in user-visible locations", and Git made the change anyway, in comments and in a parameter name in branch.h. It was a contained cleanup (3 files, ~13 lines), not a codebase-wide rename, which is roughly the shape I'm proposing here.

I went with committish rather than commit-ish because it reads better in running prose and avoids an awkward line-break hazard. Git explicitly sanctions both.

Since super db is Git-shaped, readers may arrive with Git's vocabulary already loaded, so it seems logical to match Git's spelling.

Part of why I lean toward wanting to make this change is because I hope to introduce codespell for automated spell checking in a future PR, and they correct "commitish" to "committish" in their default dictionary. If we want to stick with what we're already doing, of course I could just add an exclusion. Ours to decide.

Counterargument

In fairness, the raw numbers across all of GitHub favor our current spelling:

Spelling	Occurrences across GitHub
commitish	107,008
committish	52,096
commit-ish	30,208

But roughly 44,928 of those commitish hits are target_commitish, the field name in GitHub's Releases API, so that's one upstream API decision replicated across tens of thousands of repos.

In the Code

The word appears 118 times repo-wide: 42 in docs and the rest in Go and the PEG grammar. Claude helped me check that "commitish" does not currently leak out into help text, parse errors, or user-visible strings, so if we were ok making this change in the user-facing docs and want to let it live on as "commitish" in the code, I'm personally fine with that. But if folks that look after the code are on board with the docs change and want to be fully consistent in the code too, great.