docs: improve grammar of sentence in migration guide

[G-Rath]

Description 📖

This pull request improves the grammar of a sentence in the migration guide

Background 📜

This was happening because I was reading the guide, and noticed this sentence didn't make sense

The Fix 🔨

By changing the pluralization of the word, it now makes more sense.

I also changed the word itself as it ends up having the same number of characters and I think reads a little more naturally

Screenshots 📷