change undeclared variable to intended value

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Closes https://github.com/facebook/react-native/pull/7770

Differential Revision: D3352007

fbshipit-source-id: eedb964d245445b61fed79245380f0803473c455

Libraries/Storage/AsyncStorage.js

@@ -122,7 +122,7 @@ var AsyncStorage = {
    *  traits: {eyes: 'blue', shoe_size: 10}
    * };
 
-   * AsyncStorage.setItem(store_key, JSON.stringify(UID123_object), () => {
+   * AsyncStorage.setItem('UID123', JSON.stringify(UID123_object), () => {
    *   AsyncStorage.mergeItem('UID123', JSON.stringify(UID123_delta), () => {
    *     AsyncStorage.getItem('UID123', (err, result) => {
    *       console.log(result);