Update date-variable-formatting.md

[nep]

Overview

A little text tweaking about singular usage

Nature of This Change

• [ ] 🐛 Fixes a bug
• [ ] 🚀 Implements a new feature
• [x] 🛁 Rewrites existing documentation
• [ ] 💅 Fixes coding style
• [ ] 🔥 Removes unused files / code

Related Application Change

None