APIM Custom Properties for TripleDes is incorrect

https://github.com/Azure/azure-rest-api-specs/blob/64496bd64b0376dc4b45e3193a39f7bcdd4b28da/specification/apimanagement/resource-manager/Microsoft.ApiManagement/stable/2021-08-01/apimdeployment.json#L1355

```
Custom properties of the API Management service.
Setting `Microsoft.WindowsAzure.ApiManagement.Gateway.Security.Ciphers.TripleDes168` will disable the cipher TLS_RSA_WITH_3DES_EDE_CBC_SHA for all TLS(1.0, 1.1 and 1.2).
```

This is not correct, when you disable 3DES in APIM the custom property that gets updated is called `Microsoft.WindowsAzure.ApiManagement.Gateway.Security.Ciphers.TLS_RSA_WITH_3DES_EDE_CBC_SHA`
This should read:
```
Custom properties of the API Management service.
Setting `Microsoft.WindowsAzure.ApiManagement.Gateway.Security.Ciphers.TLS_RSA_WITH_3DES_EDE_CBC_SHA` will disable the cipher TLS_RSA_WITH_3DES_EDE_CBC_SHA for all TLS(1.0, 1.1 and 1.2).
```

Open specification/apimanagement/resource-manager/Microsoft.ApiManagement/stable/2021-08-01/apimdeployment.json and inspect the custom-properties description around line 1355. Compare the documented property with the issue's stated APIM property name, update the description accordingly, and verify that the specification remains valid.

Written by the indexing model from the issue text.