fix(pptx): correct hidden-slide comment for include_hidden_slides

[developer0hye]

File submission policy

• No sample documents, attachments, or private fixtures are submitted.

Summary

#1758 added the include_hidden_slides opt-in but left a call-site comment
in PptxParser::parse describing unconditional hidden-slide omission. This
now contradicts parse_single_slide's own updated docstring. Update the
comment to match, name both show="0" and show="false" encodings, and
restore a blank line the PR's diff had dropped for no reason.

Comment-only change; no behavior change.

Related issue

Related: #1758

Testing

• cargo fmt --all -- --check
• cargo build --locked -p office2pdf
• cargo test --locked -p office2pdf pptx

Visual impact

• No rendered PDF change
• Rendered PDF change or visual evidence added
• Reason: Comment-only change in crates/office2pdf/src/parser/pptx.rs; no drawing or layout code changed.

Checklist

• Commits include a Signed-off-by line
• PR scope contains one root cause
• Remaining converter or harness deviations each reference an open issue (none introduced)