Update sql.md to fix url.

[trevordunn]

Description of what this PR is changing or adding, and why:

Fixes a url in the docs.

Issues fixed by this PR (if any):

PRs or commits this PR depends on (if any):

Presubmit checklist

• This PR is marked as draft with an explanation if not meant to land until a future stable release.
• This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• This PR uses semantic line breaks of 80 characters or fewer.

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[sfshaza2]

/gcbrun

[flutter-website-bot]

Visit the preview URL for this PR (updated for commit e678c74):

https://flutter-docs-prod--pr11515-patch-1-x2oxoy2j.web.app

[sfshaza2]

lgtm