Change attribute mentioned in alignItem

Summary:
Since alignItem works on the secondary axis, in the example, the value heigth: 50 should be deleted so that alignItem: 'stretch' can work. Not width: 50. In fact, width needs to be there, or else no component will be shown at all.

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Closes #16109

Differential Revision: D6017969

Pulled By: shergin

fbshipit-source-id: af37b831f1bb6206dabdaff36d4cb9e88fb1a496

docs/LayoutWithFlexbox.md

@@ -73,7 +73,7 @@ AppRegistry.registerComponent('AwesomeProject', () => JustifyContentBasics);
 
 Adding `alignItems` to a component's style determines the **alignment** of children along the **secondary axis** (if the primary axis is `row`, then the secondary is `column`, and vice versa). Should children be aligned at the start, the center, the end, or stretched to fill? Available options are `flex-start`, `center`, `flex-end`, and `stretch`.
 
-> For `stretch` to have an effect, children must not have a fixed dimension along the secondary axis. In the following example, setting `alignItems: stretch` does nothing until the `width: 50` is removed from the children.
+> For `stretch` to have an effect, children must not have a fixed dimension along the secondary axis. In the following example, setting `alignItems: stretch` does nothing until the `height: 50` is removed from the children.
 
 ```ReactNativeWebPlayer
 import React, { Component } from 'react';