minor fix to decimal type docs

[ErhanAbi]

❓ Type of change

• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

I was looking on lucid's docs for the decimal column type and got confused by: Specifying null as precision creates a decimal column that can store numbers of precision and scale.. Checked on knex docs and saw that null creates a column with any precision and scale.

It's just a minor wording fix.

📝 Checklist

• I have linked an issue or discussion.
• I have updated the documentation accordingly.