Clarify the number of examples.

I didn't understand these were three different examples.

docs/rules/jsx-key.md

@@ -5,7 +5,7 @@ array literal or an arrow function expression.
 
 ## Rule Details
 
-Examples of **incorrect** code for this rule:
+Three different examples of **incorrect** code for this rule:
 
 ```jsx
 [<Hello />, <Hello />, <Hello />];