Fix documentation error for Delegated Types in Rails Guide

[ron-shinall]

Motivation / Background

The Rails Guide for Delegated Types incorrectly references delegates.

This Pull Request has been created to correct that error.

Detail

This Pull Request changes the references from delegates to delegate.

Checklist

Before submitting the PR make sure the following are checked:

• This Pull Request is related to one change. Changes that are unrelated should be opened in separate PRs.
• Commit message has a detailed description of what changed and why. If this PR fixes a related issue include it in the commit message. Ex: [Fix #issue-number]
• Tests are added or updated if you fix a bug or add a feature.
• CHANGELOG files are updated for the changed libraries if there is a behavior change or additional feature. Minor bug fixes and documentation changes should not be included.

[eileencodes]

Thank you!