The id of "userAssignedIdentities" with resource group name in lowercase returned by IotHubResource_Get API

The id of "userAssignedIdentities" with resource group name in lowercase returned by [IotHubResource_Get API ](https://github.com/Azure/azure-rest-api-specs/blob/52f9379c7709eb53f15b875b7de9b1abfa17f298/specification/iothub/resource-manager/Microsoft.Devices/stable/2021-07-02/iothub.json#L83) :

Repro steps:
1. Create a IotHub resource(the type of identity is "UserAssigned")
The resource group name in `userAssignedIdentities` is `resourceGroups` in the following request body:
```{
"identity":{
"type":"UserAssigned",
"userAssignedIdentities":{
"/subscriptions/XXX/resourceGroups/elenaTest/providers/Microsoft.ManagedIdentity/userAssignedIdentities/userAssignedIdentityTest":{
}
},
```

2. Call [IotHubResource_Get API ](https://github.com/Azure/azure-rest-api-specs/blob/52f9379c7709eb53f15b875b7de9b1abfa17f298/specification/iothub/resource-manager/Microsoft.Devices/stable/2021-07-02/iothub.json#L83) .\
The resource group name in `userAssignedIdentities` is `resourcegroups` in the following response body:
```
"identity": {
"type": "UserAssigned",
"userAssignedIdentities": {
"/subscriptions/XXX/resourcegroups/elenaTest/providers/Microsoft.ManagedIdentity/userAssignedIdentities/userAssignedIdentityTest": {
"clientId": "XXX",
"principalId": "XXX"
}
}
}
}
```

Expected:
The resource group name in `userAssignedIdentities` is `resourceGroups`

Actual:
The resource group name in `userAssignedIdentities` is `resourcegroups`

Start with specification/iothub/resource-manager/Microsoft.Devices/stable/2021-07-02/iothub.json at the IotHubResource_Get definition and compare the documented identity key with the reported response. Reproduce the request if possible; done means the returned userAssignedIdentities resource-group segment retains the expected casing.

Written by the indexing model from the issue text.