Fix TypeScript type error in example code

Without the `| null`, the following error appears during compilation:
```
TS2322: Type RefObject<HTMLDivElement | null> is not assignable to type RefObject<HTMLDivElement>
Type HTMLDivElement | null is not assignable to type HTMLDivElement
Type null is not assignable to type HTMLDivElement
```

Author: martinfrancois

docs/01-app/03-api-reference/03-file-conventions/error.mdx

@@ -219,7 +219,7 @@ export class GracefullyDegradingErrorBoundary extends Component<
   ErrorBoundaryProps,
   ErrorBoundaryState
 > {
-  private contentRef: React.RefObject<HTMLDivElement>
+  private contentRef: React.RefObject<HTMLDivElement | null>
 
   constructor(props: ErrorBoundaryProps) {
     super(props)