docs: Fix keyboard shortcut for strikethrough

[cmayoracurzio]

Please describe your changes

Simple PR to fix docs, with regards to the keyboard shortcut for strikethrough text formatting.

How did you accomplish your changes

Simple markdown editing of keyboard-shortcuts.md.

How have you tested your changes

Tested manually with Tiptap editor in React and Next.js projects.

How can we verify your changes

Check the keyboard shortcuts that are defined in strike extension source code.

Checklist

• [x] The changes are not breaking the editor
• [ ] Added tests where possible
• [x] Followed the guidelines
• [ ] Fixed linting issues

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