docs: change Expo installation command (#856)

Follow Expo's recommendation to use `npx`

Author: fmcalado

website/docs/Installation.md

@@ -18,7 +18,7 @@ yarn add @react-native-async-storage/async-storage
 
 With Expo CLI:
 ```bash
-expo install @react-native-async-storage/async-storage
+npx expo install @react-native-async-storage/async-storage
 ```
 
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