Fix `placeholder` selector for form components

[langermank]

Placeholder text came up during the v8 color staff ship. It looks like the selector wasn't properly selecting, which could lead to unexpected results.

Rollout strategy

• [ ] Patch release
• [ ] Minor release
• [ ] Major release; if selected, include a written rollout or migration plan
• [ ] None; if selected, include a brief description as to why

Testing & Reviewing

Merge checklist

• [ ] Added/updated tests
• [ ] Added/updated documentation
• [ ] Added/updated previews (Storybook)
• [ ] Changes are SSR compatible
• [ ] Tested in Chrome
• [ ] Tested in Firefox
• [ ] Tested in Safari
• [ ] Tested in Edge
• [ ] (GitHub staff only) Integration tests pass at github/github (Learn more about how to run integration tests)

[changeset-bot[bot]]

🦋 Changeset detected

Latest commit: 853bda0d147371b18d0d9bbcae4d8ea0618cda51

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@primer/react	Patch

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[github-actions[bot]]

size-limit report 📦

Path	Size
packages/react/dist/browser.esm.js	113.48 KB (+0.02% 🔺)
packages/react/dist/browser.umd.js	114.16 KB (+0.02% 🔺)