fix typo (close #1368)

examples/modal/modal.css

@@ -41,7 +41,7 @@
 
 /*
  * the following styles are auto-applied to elements with
- * v-transition="modal" when their visiblity is toggled
+ * v-transition="modal" when their visibility is toggled
  * by Vue.js.
  *
  * You can easily play with the modal transition by editing