docs: Fix grammar errors in collections.md

- Remove incorrect article:
  "the most relational databases" → "most relational databases"
- Fix subject-verb agreement:
  "it gather" → "it gathers"
- Fix noun-verb agreement:
  "Cursor and counter" → "Cursors and counters" (2 instances)

[ci skip]

Co-Authored-By: Claude <noreply@anthropic.com>