az notification-hub namespace check-availability typo

**Describe the bug**
When checking availability of a notifications-hub namespace, the property "isAvailable" is misspelled. Currently, the property is returned as "isAvailiable".

**To Reproduce**
Install the az CLI.
Run the following powershell script:
```
az login
az config set extension.use_dynamic_install=yes_without_prompt
$result = az notification-hub namespace check-availability -n "ntfns-example-namespace-name" | ConvertFrom-Json
Write-Host $result.isAvailable
Write-Host $result.isAvailiable
```

**Expected behavior**
For this script, the first Write-Host command should output "True" or "False", depending on the availability of the example namespace. The second Write-Host command should output an empty string. Because the variable is misspelled, the opposite occurs.

**Environment summary**
az cli was installed on a windows environment using the MSI installer
az --version outputs:
azure-cli 2.21.0
core 2.21.0
telemetry 1.0.6
notification-hub 0.2.0
Python (Windows) 3.6.8 (tags/v3.6.8:3c6b436a57, Dec 23 2018 23:31:17) [MSC v1916 32 bit (Intel)]

notification-hubs command group was installed via the dynamic extension install option
OS is Windows 10
Shell is Powershell