Fix grammer for the Safety documentation check
The original message ("unsafe function's docs miss `# Safety` section") reads quite awkwardly. I've changed it to "unsafe function's docs are missing a `# Safety` section" to have it read better.
```
changelog: [`missing_headers`]: Tweak the grammar in the lint message
```
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| .. | ||
| test_utils | ||
| ui | ||
| ui-cargo | ||
| ui-internal | ||
| ui-toml | ||
| workspace_test | ||
| check-fmt.rs | ||
| clippy.toml | ||
| compile-test.rs | ||
| dogfood.rs | ||
| headers.rs | ||
| integration.rs | ||
| lint_message_convention.rs | ||
| missing-test-files.rs | ||
| versioncheck.rs | ||
| workspace.rs | ||