Issue Report: /en-us/sponsored-brands/3-0/openapi

If you need help troubleshooting a problem with the Amazon Ads API, visit the [Amazon Ads API service desk](https://amzn-clicks.atlassian.net/servicedesk/customer/portal/2/group/2). Issues are for reporting documentation bugs and requesting enhancements only.

**Describe the bug**
The endpoint for `/sb/themes/list` is not correct.

**URL of the content**
https://advertising.amazon.com/API/docs/en-us/sponsored-brands/3-0/openapi#tag/Theme-targeting/operation/sbListThemes

**Description of what the documentation currently says**
The documentation says that it's meant to consume the content-type `application/vnd.sbthemeslistrequest.v3+json` and return the content type `application/vnd.sbthemeslistresponse.v3+json`. However, when calling with those content types, I always get a 415 error, which implies that it's supposed to be some other MIME type. Sooooo, which is it?

Parenthetically, making the APIs work only with these weird vendor MIME types has only caused pain + heartache.