It should be `categories` not `category` in http request body and http response body in Security.AssessmentMetadata API

After tested, seems in Security.AssessmentMetadata API doesn't make [`category`](https://github.com/Azure/azure-rest-api-specs/blob/9a5ad06be61902cdbbc44bee02084a07a3a72499/specification/security/resource-manager/Microsoft.Security/stable/2020-01-01/assessmentMetadata.json#L317) take effect. After confirmed with service team, they said It should be `categories` not `category` in http request body and http response body. So they intend to fix it asap.

Open specification/security/resource-manager/Microsoft.Security/stable/2020-01-01/assessmentMetadata.json at the AssessmentMetadata schema around line 317 and inspect the request and response body definitions. Done means the field is named `categories` rather than `category` in both bodies, matching the service team's correction.

Written by the indexing model from the issue text.