Fix typo, grammar, and choice of words

[PaperMonster]

1. Fix typo
2. Fix grammar issues
3. Choice of words revised
4. Capitalization
5. Change "คนที่มีความเสี่ยง (COVID-19 Risk person)" to "ผู้ติดเชื้อ (COVID-19 positive person)", as the former is ambiguous and misleading. AFAIK, only users who are diagnosed with COVID-19 are the origin of contract tracing. Correct me if I'm wrong.
6. Change statements along the line of "notify you immediately if you are close to a risky person" to past tense, because the app only notifies user after a case is confirmed, not right at the moment of contacts.

If 5. and 6. are accepted, other languages need to be updated as well.