docs: Fix MaterialToolkitTheme namespace (#500)

* docs: Fix `MaterialToolkitTheme` namespace

doc/material-getting-started.md

@@ -44,7 +44,7 @@ Initialization of the Material Toolkit resources is handled by the specialized `
         <ResourceDictionary.MergedDictionaries>
 
             <!-- Load Material Toolkit resources (also loads the base Uno.Material resources) -->
-            <MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI" />
+            <MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI.Material" />
 
             <!-- Load custom application resources -->
             <!-- ... -->
@@ -80,7 +80,7 @@ If your application is based on the older solution template that includes a shar
                     <XamlControlsResources xmlns="using:Microsoft.UI.Xaml.Controls" />
 
                     <!-- Load Material Toolkit resources (also loads the base Uno.Material resources) -->
-                    <MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI" />
+                    <MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI.Material" />
 
                     <!-- Load custom application resources -->
                     <!-- ... -->
@@ -122,4 +122,4 @@ In `AppResources.xaml`, instead of initializing `MaterialFonts`, you would use t
 - [Uno Platform Material Toolkit Sample App](https://github.com/unoplatform/Uno.Samples/tree/master/UI/UnoMaterialToolkitSample)
 - [Uno Platform Material Toolkit Figma File](https://www.figma.com/community/file/1110792522046146058)
 - [Official Material Design 3 Guidelines](https://m3.material.io/components)
-- [Official Material Design 3 Theme Builder](https://m3.material.io/theme-builder)
+- [Official Material Design 3 Theme Builder](https://m3.material.io/theme-builder)