Update jsx-key.md

Fix bugs in rendering an array of elements to prevent from unintentional copying it.

Author: Adam Gołąb

docs/rules/jsx-key.md

@@ -10,15 +10,15 @@ The following patterns are considered warnings:
 ```jsx
 [<Hello />, <Hello />, <Hello />];
 
-data.map(x => <Hello>x</Hello>);
+data.map(x => <Hello>{x}</Hello>);
 ```
 
 The following patterns are **not** considered warnings:
 
 ```jsx
 [<Hello key="first" />, <Hello key="second" />, <Hello key="third" />];
 
-data.map((x, i) => <Hello key={i}>x</Hello>);
+data.map((x, i) => <Hello key={i}>{x}</Hello>);
 ```
 
 ## When not to use