Link for 'Transactional Templates' in use_cases README.md broken

Issue Summary

The link for transactional templates is broken in use_cases README.md due to a typo in the link.

Steps to Reproduce

1. Navigate to: https://github.com/sendgrid/sendgrid-python/blob/master/use_cases/transactional_templates.md
2. Click 'Transactional Templates' link

The link is set to:
https://github.com/sendgrid/sendgrid-python/blob/master/use_cases/transational_templates.md

It should be:
https://github.com/sendgrid/sendgrid-python/blob/master/use_cases/transactional_templates.md

Technical details:

• sendgrid-python Version: 6.0.4
• Python Version: 3.7.2

[thinkingserious]

Thank you! I have fixed the link.