docs: word correction (#389)

correcting the word "exiting" to "existing"

Author: mRiloB

website/docs/Usage.md

@@ -17,7 +17,7 @@ import AsyncStorage from '@react-native-community/async-storage';
 
 ### Storing data
 
-`setItem()` is used both to add new data item (when no data for given key exists), and to modify exiting item (when previous data for given key exists).
+`setItem()` is used both to add new data item (when no data for given key exists), and to modify existing item (when previous data for given key exists).
 
 #### Storing string value
 ```jsx