Update input-group.md with form text example

[ryanmortier]

Description

This updated example shows where to correctly put form text (i.e. not inside the input group).

Motivation & Context

I was putting the form text elements inside the input-group until I realized this was wrong. There doesn't seem to be any examples indicating where to put this.

Type of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (non-breaking change)
• Breaking change (fix or feature that would change existing functionality)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• My change introduces changes to the documentation
• I have updated the documentation accordingly
• I have added tests to cover my changes
• All new and existing tests passed

Live previews

• https://deploy-preview-37406--twbs-bootstrap.netlify.app/docs/5.2/forms/input-group/