docs: Update documentation for retrieving all templates

[tony-ho]

Fixes

Fixes #759

Checklist

• I have made a material change to the repo (functionality, testing, spelling, grammar)
• I have read the [Contribution Guide] and my PR follows them.
• I updated my branch with the master branch.
• I have added tests that prove my fix is effective or that my feature works
• I have added necessary documentation about the functionality in the appropriate .md file
• I have added in line documentation to the code I modified

Short description of what this PR does:

• Add generations URI parameter to USAGE.md to demonstrate how to use this API call to retrieve all templates (dynamic & legacy)
• Add generations URI parameter to corresponding /example file to demonstrate how to use this API call to retrieve all templates (dynamic & legacy)

[SendGridDX]

All committers have signed the CLA.

[thinkingserious]

Hello @tony-ho,

Thanks again for the PR!

We appreciate your contribution and look forward to continued collaboration. Thanks!

Team SendGrid DX