docs: fix signal condition parameter

[nightcityblade]

Fixes #3176

Summary

• Correct the signal registration examples to use the supported singular condition keyword.
• Keep the decorator and add_signal examples consistent with the public API.

Testing

• python -m pytest -q tests/test_signals.py (49 passed)
• Rendered guide/content/en/guide/advanced/signals.md with the guide Markdown renderer
• ruff check sanic/mixins/signals.py
• ruff format sanic/mixins/signals.py --check
• git diff --check

[nightcityblade]

CI note: the test matrix stopped on two pre-existing mypy errors in sanic/pages/error.py (lines 15 and 43), which are unrelated to this documentation-only change. The same errors are present in the repository's earlier test run 29780572243. The remaining matrix jobs were cancelled by fail-fast, and the coverage job then failed because no coverage.xml artifact was produced.

The focused local validation for this change remains green: 49 signal tests passed, the guide page rendered successfully, and the relevant Ruff checks passed.

[nightcityblade]

CI note: this PR changes only guide/content/en/guide/advanced/signals.md. The type-check job fails in unchanged sanic/pages/error.py (lines 15 and 43), and the coverage job then fails because no coverage.xml artifact was produced. The same two checks are failing on #3177 from the same base commit (5ffc7b3), so these failures are unrelated to this documentation change.

The focused signal suite and documentation rendering listed in the PR description pass locally.