Set blog_search_field_placeholder i18n key

[takakd]

Pull Request type

Please check the type of change your PR introduces:

• [x] Bug-fix
• [ ] Feature (functionality, design, translations, etc.)
• [ ] Documentation change
• [ ] Project management (tests, CI, GitHub configuration, etc.)
• [ ] Other (please describe):

Current state

Issue Number(s): #424

Proposed changes

Set the key to in the search input placeholder.

Screenshots, if applicable

Checklist

• [x] Bug-fixes and new features: I have tested locally with the latest release of Hugo extended. This requirement is a standard.
• [ ] Bug-fixes, new features, and doc changes: I have updated the relevant documentation as part of this PR.
• [x] All PRs: I have signed off (using git commit -s ...), or if not possible due to developer environment constraints, will comment below confirming that I am adhering to the Developer Certificate of Origin.