Fix `MarkdownTextInput` React ref type

[tomekzaw]

Currently, MarkdownTextInput component in example app has a ref of type TextInput.

react-native-live-markdown/example/src/App.tsx

Lines 34 to 35 in 5638b54

	// TODO: use MarkdownTextInput ref instead of TextInput ref
	const ref = React.useRef<TextInput>(null);

The objective is to change React ref type from TextInput to MarkdownTextInput.

-React.useRef<TextInput>(null); 
+React.useRef<MarkdownTextInput>(null);