Fix missing apostrophe in API docs

[travis-g]

Description

Simply a docs change to possessiveness to a word.

Motivation and Context

I'm proofreading while getting my dev environment set up. We should probably open a generic "docs" issue label.

How Has This Been Tested?

Technically no.

Screenshots (if appropriate):

Types of changes

• [x] Documentation enhancement
• [ ] 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.
• [x] My change requires a change to the documentation.
• [x] I have updated the documentation accordingly.
• [ ] I have read the CONTRIBUTING document.

PS: I don't think you have a contributing document?