[Ch8] Prevent Loss of Focus in Example

Adds a key to the form element, preventing unnecessary rerenders that drop input focus whenever the error div is added to (or removed from) the DOM.

Author: j-wenning

exercises/chapter8/src/Main.purs

@@ -127,6 +127,7 @@ mkAddressBookApp =
                                 ]
                               <> renderPhoneNumbers
                           ]
+                      , key: "person-form"
                       }
                   ]
           }