updatemenus: fix 'yanchor' dflr

- 'bottom' -> 'top'
- 'bottom' was misplaced in #770 when the attributes
   were ported over from range selectors

Author: etpinard

src/components/updatemenus/attributes.js

@@ -100,7 +100,7 @@ module.exports = {
     yanchor: {
         valType: 'enumerated',
         values: ['auto', 'top', 'middle', 'bottom'],
-        dflt: 'bottom',
+        dflt: 'top',
         role: 'info',
         description: [
             'Sets the update menu\'s vertical position anchor',