Remove docs reference to incorrect align-items default

Summary:
Per https://developer.mozilla.org/en-US/docs/Web/CSS/align-items, the default for `align-items` is stretch, so I removed the reference that suggested otherwise

Similar to facebook#9397, but this section was not updated in that PR.
Closes facebook#11373

Differential Revision: D4299871

Pulled By: ericnakagawa

fbshipit-source-id: 1bee27b0dced192ef42e21ca8cc800d52edce195

docs/LayoutWithFlexbox.md

@@ -12,7 +12,7 @@ A component can specify the layout of its children using the flexbox algorithm.
 
 You will normally use a combination of `flexDirection`, `alignItems`, and `justifyContent` to achieve the right layout.
 
-> Flexbox works the same way in React Native as it does in CSS on the web, with a few exceptions. The defaults are different, with `flexDirection` defaulting to `column` instead of `row`, and `alignItems` defaulting to `stretch` instead of `flex-start`, and the `flex` parameter only supports a single number.
+> Flexbox works the same way in React Native as it does in CSS on the web, with a few exceptions. The defaults are different, with `flexDirection` defaulting to `column` instead of `row`, and the `flex` parameter only supporting a single number.
 
 #### Flex Direction