Replace onchange with EventListener method (#773)

The MDN example that redirects to this code uses EventListener method to
detect change in select element. I learned that standard practice is
using EventListener method, hence the change.

javascript/building-blocks/simple-else-if.html

@@ -21,7 +21,7 @@
       const select = document.querySelector('select');
       const para = document.querySelector('p');
 
-      select.onchange = setWeather;
+      select.addEventListener("change", setWeather);
 
       function setWeather() {
         const choice = select.value;