Improve keys example

The previous example about hiding keys had syntax errors, and example in itself didn't follow our own guidelines (key should be on immediate child of an array)

In the new example we reuse the code from the previous example but break out a User component (which is much more likely) and the example says to move the key from inside the user component instead

docs/keys.md

@@ -96,21 +96,21 @@ users.map(function(u) {
 
 #### Avoid hiding keys in component root elements
 
-If you refactor the code and put the button inside a component, the key must be moved out of the component and placed back where the component took the place of the button.
+If you refactor the code and make a user component, the key must be moved out of the component and put on the component itself, since it is now the immediate child of the array.
 
 ```javascript
 // AVOID
-var Button = {
+var User = {
 	view: function(vnode) {
-		return m("button", {key: vnode.attrs.id}, u.name)
+		return m("div", { key: vnode.attrs.user.id }, [
+      m(Button, vnode.attrs.user.name)
+    ])
 	}
 }
 
 // PREFER
 users.map(function(u) {
-	return m("div", [
-		m(Button, {key: u.id}, u.name) // key should be here, not in component
-	])
+	return m(User, { key: u.id, user: u }) // key should be here, not in component
 })
 ```
 
@@ -195,12 +195,12 @@ users[0].key = 'c'
 // AVOID
 users.map(function(user){
 	// The component for John will be destroyed and recreated
-	return m(UserComponent, user) 
+	return m(UserComponent, user)
 })
 
 // PREFER
 users.map(function(user){
 	// Key is specifically extracted: data model is given its own property
-	return m(UserComponent, {key: user.id, model: user}) 
+	return m(UserComponent, {key: user.id, model: user})
 })
 ```