Fix doc with right unmounting call (#1151)

## Description
Prevented a bug in documentation where during unmounting the listeners
were not being cleared.

### Why
What is the motivation for this change? Add a few sentences describing
the context and overall goals of the pull request's commits.

Resolves #997 

## Screenshots
Add any relevant screen captures here from before or after your changes.

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/react-native-windows-samples/pull/1151)

Co-authored-by: Harini Malothu <hmalothu@microsoft.com>

Author: HariniMalothu17

docs/windowsbrush-and-theme.md

@@ -55,11 +55,11 @@ class MyAppClass extends Component {
   };
 
   componentDidMount() {
-    Appearance.addChangeListener(this.onAppThemeChanged);
+    this._subscription = Appearance.addChangeListener(this.onAppThemeChanged);
   };
 
   componentWillUnmount() {
-    Appearance.addChangeListener(this.onAppThemeChanged);
+    this._subscription?.remove();
   };
 
   onAppThemeChanged = (theme) => {