docs: Fix MaterialToolkitTheme namespace

[Youssef1313]

GitHub Issue (If applicable): #

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Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Docs have been added/updated which fit documentation template. (for bug fixes / features)
• Unit Tests and/or UI Tests for the changes have been added (for bug fixes / features) (if applicable)
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• Contains NO breaking changes
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Other information

Internal Issue (If applicable):

[kazo0]

There are a couple other spots that we need to fix the namespace in that file

uno.toolkit.ui/doc/material-getting-started.md

Line 47 in 3edaf9f

<MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI" />

uno.toolkit.ui/doc/material-getting-started.md

Line 83 in 3edaf9f

<MaterialToolkitTheme xmlns="using:Uno.Toolkit.UI" />