There is a malformed http link in the Partition definition section on the Partition-Based Sync page, just below the image that demonstrates the partition key/value as a color. You can see the destination of the link, which itself may not be correct, since it takes you to the https://www.mongodb.com/use-cases/mobile#atlas-form-container page when clicked.

Link: https://www.mongodb.com/docs/atlas/app-services/reference/partition-based-sync/#partition