Fix README italic markdown documentation

[Copilot]

The README incompletely documented italic markdown conversion, showing only *text* → _text_ while the code also handles _text_ (already valid Mrkdwn).

Changes

• README.md: Clarified italic conversion supports both *text* and _text_, noting underscores are already in Mrkdwn format
• Code formatting: Applied go fmt to remove trailing whitespace

Before/After

Before:

- **Italic**: `*text*` → `_text_`

After:

- **Italic**: `*text*` or `_text_` → `_text_` (single underscores already in Mrkdwn format)

Original prompt

Please check the README is correct compared to the code.

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