Update docs url in the README for signature

[leonardomendix]

Pull request type

No code changes (changes to documentation, CI, metadata, etc.)

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Description

Updated link to the one pointing to the signature widget in the docs site.

[github-actions]

AI Code Review

✅ Approved — no issues found

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What was reviewed

File	Change
packages/pluggableWidgets/signature-web/README.md	Removed stale TODO comment; updated docs link from generic widgets URL to widget-specific /signature path

Skipped (out of scope): N/A — single file, no dist/, no lockfiles

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Positives

• Correctly resolves the long-standing TODO: Update marketplace URL comment left over from initial scaffolding
• The new URL https://docs.mendix.com/appstore/widgets/signature points directly to the widget's documentation page rather than the generic index
• No changelog entry needed — docs-only change with no runtime, XML, or behavior impact
• PR description clearly states what changed and why