fix: event declartion and handler (#929)

## Description

### Why
* 'ReactEvent' is obsolete, it raises error when compiling, change it to
'Action'.
* 'eventHandler' is a member of the component, it raises render error
without 'this.'

## Screenshots
<img width="378" alt="ReactEvent error"
src="https://github.com/microsoft/react-native-windows-samples/assets/4596887/35213737-c374-4c0b-8ea4-0da16cb86ae2">

<img width="459" alt="ReactEvent Obsolete"
src="https://github.com/microsoft/react-native-windows-samples/assets/4596887/3661add7-508f-4808-b742-190a3c625ad0">

<img width="385" alt="eventHandler not defined"
src="https://github.com/microsoft/react-native-windows-samples/assets/4596887/99a9b1c4-4c5e-4bb9-a860-b11da9031a2d">

###### Microsoft Reviewers: [Open in
CodeFlow](https://microsoft.github.io/open-pr/?codeflow=https://github.com/microsoft/react-native-windows-samples/pull/929)

Co-authored-by: Jon Thysell <jthysell@microsoft.com>

Author: rabbitom

docs/native-modules.md

@@ -155,7 +155,7 @@ namespace NativeModuleSample
     }
 
     [ReactEvent]
-    public ReactEvent<double> AddEvent { get; set; }
+    public Action<double> AddEvent { get; set; }
   }
 }
 ```
@@ -503,12 +503,12 @@ class NativeModuleSample extends Component {
 
   componentDidMount() {
     // Subscribing to FancyMath.AddEvent
-    FancyMathEventEmitter.addListener('AddEvent', eventHandler, this);
+    FancyMathEventEmitter.addListener('AddEvent', this.eventHandler, this);
   }
 
   componentWillUnmount() {
     // Unsubscribing from FancyMath.AddEvent
-    FancyMathEventEmitter.removeListener('AddEvent', eventHandler, this);
+    FancyMathEventEmitter.removeListener('AddEvent', this.eventHandler, this);
   }
 
   eventHandler(result) {