docs: fix okta-tile refs

[opis-mark]

Description

Corrects the README.md's use of okta-aws-tile to okta-tile which is the real parameter name.

Related Issue

Resolves #157

Motivation and Context

The documentation and examples should be accurate to best help those using it.

How Has This Been Tested?

This is only an update to documentation so no testing was done

Types of changes

• 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)
• Documentation update

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[opis-mark]

it looks like maybe the OS for the tox tests changed and the newer OS doesn't support python 3.7. not sure if we want to continue supporting 3.7 or not