docs: fix typo and use HTTPS for CommonMark spec links

[deeferentleeg]

Summary

A few small documentation fixes across docs/:

• docs/performance.md: fix a typo in footnote [^2] — mistletoes's → mistletoe's. The project being referenced is named mistletoe (singular), so the possessive was incorrectly formed with an extra s. (The unrelated it's → its fix on the same file is already handled in docs: Fix grammar error in performance.md #402, so it is intentionally left untouched here to avoid duplicating that change.)
• docs/index.md and docs/using.md: update the CommonMark specification link from http://spec.commonmark.org/ to https://spec.commonmark.org/. The site serves over HTTPS and this keeps the docs consistent with the HTTPS links already used elsewhere in the project (e.g. the GitHub Flavored Markdown link in using.md).

These are documentation-only changes; no code or tests are affected.

Notes

• There is no corresponding open issue for these; they are minor docs/typo corrections spotted while reading the documentation. Happy to open an issue if the maintainers prefer one linked.
• I checked that no existing open PR already covers the mistletoe's typo or these http → https spec-link updates (docs: Fix grammar error in performance.md #402 only touches the it's/its line).

AI usage disclosure

An AI assistant was used to help locate the relevant code and draft this change. I have reviewed and understand every line of the diff, and I am the sole author of this contribution.