fixed typo

src/plots/cartesian/attributes.js

@@ -33,7 +33,7 @@ module.exports = {
             'a 2D cartesian y axis.',
             'If *y* (the default value), the y coordinates refer to',
             '`layout.yaxis`.',
-            'If *y2*, the y coordinates refer to `layout.xaxis2`, and so on.'
+            'If *y2*, the y coordinates refer to `layout.yaxis2`, and so on.'
         ].join(' ')
     }
 };