Fixed props.md example on Greetings component (#552)

Greetings component example in procs.md didnn't have enclosing jsx tag and thus failed, this is to fix that.

Author: abhishekkr

docs/props.md

@@ -37,7 +37,9 @@ import { AppRegistry, Text, View } from 'react-native';
 class Greeting extends Component {
   render() {
     return (
-      <Text>Hello {this.props.name}!</Text>
+      <View style={{alignItems: 'center'}}>
+        <Text>Hello {this.props.name}!</Text>
+      </View>
     );
   }
 }