Fix undefined variable in python3-http-debian template

[welteki]

Description

The format_response function uses an undefined variable resp this should be res. This bug causes functions using the http-debian template to fail.

Motivation and Context

• [ ] I have raised an issue to propose this change (required)
• [ ] My issue has received approval from the maintainers or lead with the design/approved label

Functions using the latest version of the http-debian template would fail with a status 500 this resolves that issue

How Has This Been Tested?

Verified deploying a function build using the updated template returns a 200 status code when invoked.

Types of changes

• [x] Bug fix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• [x] My code follows the code style of this project.
• [ ] My change requires a change to the documentation.
• [ ] I have updated the documentation accordingly.
• [x] I've read the CONTRIBUTION guide
• [x] I have signed-off my commits with git commit -s
• [ ] I have added tests to cover my changes.
• [ ] All new and existing tests passed.