doc: rename references to xsl instead of xls

[sxppro]

XSL is the stylesheet for XML, XLS is a spreadsheet file format for Microsoft Excel

🔗 Linked issue

❓ Type of change

• 📖 Documentation (updates to the documentation or readme)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

Replace references to XLS, a spreadsheet file format, with XSL, the stylesheet for XML to reduce confusion.

[harlan-zw]

Thanks!