Mock API: Fixed invalid range for duration error message typo for Poll Post Endpoint

[MJRLegends]

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

Problem/Feature

Nothing big, just fixing a typo in the mock api errors for the Create Poll/Post Poll Endpoint

Description of Changes:

• Changed duation -> duration

Checklist

• [Yes ] My code follows the Contribution Guide
• [Yes ] I have self-reviewed the changes being requested
• [ Yes] I have made comments on pieces of code that may be difficult to understand for other editors
• [ Yes] I have updated the documentation (if applicable)