The "MarkdownTextBlock" control is now present in the Toolbox

The MarkdownTextBlock's Assembly entry in the VS manifest is wrong, which is why the control was not present in the Toolbox.
By simply updating to the correct Assembly Name lets the "MarkdownTextBlock" control to be validated and listed in the Visual Studio Toolbox.

Author: Nirmal4G

Microsoft.Toolkit.Uwp.UI.Controls.Markdown/VisualStudioToolsManifest.xml

@@ -1,5 +1,5 @@
 <FileList>
-  <File Reference="Microsoft.Toolkit.Uwp.UI.Controls.MarkdownTextBlock.dll">
+  <File Reference="Microsoft.Toolkit.Uwp.UI.Controls.Markdown.dll">
     <ToolboxItems VSCategory="Windows Community Toolkit" BlendCategory="Windows Community Toolkit">
       <Item Type="Microsoft.Toolkit.Uwp.UI.Controls.MarkdownTextBlock" />
     </ToolboxItems>