Fix grammar and wording in README files

## Summary
Fixes grammar and wording issues in README files.

## Changes
- `01/README.md`
  - `Volatile used to indicate` → `Volatile is used to indicate`
  - `i.e,` → `i.e.,`
  - `compiler doesn't` → `the compiler doesn't`
- `04/README.md`
  - `python Decimal package` → `Python Decimal package`
  - `use at your own caution` → `use with caution`

## Testing
- Documentation-only change.