Update Modal.js

Summary:
you don't need curly bracket. `{}`

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Closes #15198

Differential Revision: D5497867

Pulled By: javache

fbshipit-source-id: a09f06aabc6ea16f0b0eec12bf910ffcab804eb0

Libraries/Modal/Modal.js

@@ -47,7 +47,7 @@ const RCTModalHostView = requireNativeComponent('RCTModalHostView', null);
  *     return (
  *       <View style={{marginTop: 22}}>
  *         <Modal
- *           animationType={"slide"}
+ *           animationType="slide"
  *           transparent={false}
  *           visible={this.state.modalVisible}
  *           onRequestClose={() => {alert("Modal has been closed.")}}