Fix grammar in List and Keys documentation (#854)

content/docs/lists-and-keys.md

@@ -280,7 +280,7 @@ function NumberList(props) {
 }
 ```
 
-JSX allows [embedding any expressions](/docs/introducing-jsx.html#embedding-expressions-in-jsx) in curly braces so we could inline the `map()` result:
+JSX allows [embedding any expression](/docs/introducing-jsx.html#embedding-expressions-in-jsx) in curly braces so we could inline the `map()` result:
 
 ```js{5-8}
 function NumberList(props) {