converted placeholder input to a textarea (#133)

assets/javascripts/discourse/templates/components/wizard-custom-field.hbs

@@ -118,9 +118,8 @@
     </div>
 
     <div class="setting-value">
-      {{input
-        type="text"
-        class="medium"
+      {{textarea
+        name="field_placeholder"
         value=field.placeholder}}
     </div>
   </div>