Fix invalid memory store type: Disk (microsoft#570)

### Motivation and Context

- This fixes microsoft#562

### Description

- Fixes the Error: 500: System.ArgumentException: 
Invalid memory store type: Disk

### Contribution Checklist

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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows the [Contribution
Guidelines](https://github.com/microsoft/chat-copilot/blob/main/CONTRIBUTING.md)
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raises no violations
- [ ] All unit tests pass, and I have added new tests where possible
- [ ] I didn't break anyone 😄

webapi/Options/MemoryStoreType.cs

@@ -63,7 +63,7 @@ public static MemoryStoreType GetMemoryStoreType(this SemanticMemoryConfig memor
                 {
                     return MemoryStoreType.Volatile;
                 }
-                else if (type.Equals("TextFile", StringComparison.OrdinalIgnoreCase))
+                else if (type.Equals("Disk", StringComparison.OrdinalIgnoreCase))
                 {
                     return MemoryStoreType.TextFile;
                 }