Quickstart-bicep-fix (#139)

## Purpose
Change the asset config in the quickstart

## Does this introduce a breaking change?
<!-- Mark one with an "x". -->
```
[ ] Yes
[x] No
```

## Pull Request Type
What kind of change does this Pull Request introduce?

<!-- Please check the one that applies to this PR using "x". -->
```
[ ] Bugfix
[ ] Feature
[ ] Code style update (formatting, local variables)
[ ] Refactoring (no functional changes, no api changes)
[x] Documentation content changes
[ ] Other... Please describe:
```

Author: dominicbetts

samples/quickstarts/quickstart.bicep

@@ -136,7 +136,7 @@ resource asset 'Microsoft.DeviceRegistry/namespaces/assets@2025-07-01-preview' =
             target: 'Mqtt'
             configuration: {
               topic: 'azure-iot-operations/data/oven'
-              retain: 'Keep'
+              retain: 'Never'
               qos: 'Qos1'
             }
           }