fix typos in constants.ts, Form.tsx

[HanJaeJoon]

Reasons for making this change

fix typos

• RJSF_ADDITONAL_PROPERTIES_FLAG => RJSF_ADDITIONAL_PROPERTIES_FLAG
• acceptcharset => acceptCharset

Checklist

• [ ] I'm updating documentation
  • [ ] I've checked the rendering of the Markdown text I've added
• [x] I'm adding or updating code
  • [ ] I've added and/or updated tests. I've run npm run test:update to update snapshots, if needed.
  • [ ] I've updated docs if needed
  • [x] I've updated the changelog with a description of the PR
• [ ] I'm adding a new feature
  • [ ] I've updated the playground with an example use of the feature

[HanJaeJoon]

@heath-freenome I applied all your suggestions. I didn't thought of backwards compatibility, thanks!