**Update the sandbox guide to use the documented File content parameter and fix a minor wording issue in the Manifest table.**

### Changes:
- Replace `File(text="internal notes")` with `File(content=b"internal notes")` in the Permissions example.
- Change "What files and folder should be present..."
  to "What files and folders should be present..."

## Rationale:
The sandbox guide currently uses the `text` parameter in the
Permissions example, while sandbox examples throughout the
repository consistently construct File entries using the
`content` field with byte literals (e.g. `content=b"..."`).

Aligning the guide with existing examples makes the
documentation consistent with established sandbox manifest
usage patterns and reduces confusion when following examples
across the documentation and example code.

This change is documentation-only and does not affect runtime
behavior.

Update the Manifest row in the sandbox guide to use the correct plural form:

- Change "What files and folder should be present..." to "What files and folders should be present..."

This is a documentation-only change that improves grammar and clarity without affecting functionality.