Fix incorrect unload log message in C# SDK (#609)

## Summary
Fixes an incorrect informational log message in the C# SDK's web unload
path.

## Changes
- Updated `WebUnloadModelAsync` in
`sdk/cs/src/Detail/ModelLoadManager.cs`
- Changed the success log text from "loaded successfully" to "unloaded
successfully"

## Why
Consumers of `sdk/cs` provide their own `ILogger`, so this message is
surfaced in their application logs. The old text was misleading and
could cause confusion during debugging and support investigations.

Author: DavidLuong98

sdk/cs/src/Detail/ModelLoadManager.cs

@@ -157,7 +157,7 @@ private async Task WebUnloadModelAsync(string modelId, CancellationToken? ct = n
         }
 
         var content = await response.Content.ReadAsStringAsync(ct ?? CancellationToken.None).ConfigureAwait(false);
-        _logger.LogInformation("Model {ModelId} loaded successfully from {WebService}: {Message}",
+        _logger.LogInformation("Model {ModelId} unloaded successfully from {WebService}: {Message}",
                                modelId, _externalServiceUrl, content);
     }