Docs example for Popup

[nickforddev]

There is a minor bug in the example code for Popups:

https://soal.github.io/vue-mapbox/guide/markers&popups.html#popup

<MglPopup :coordinates="coordinates" :anchor="top">
  <VCard> <div>Hello, I'm popup!</div> </VCard>
</MglPopup>

either should be anchor="top" or :anchor="'top'", otherwise top must be a member of the component.

[nickforddev]

Also, as pointed out in #55, coordinates should not be passed by default, that was confusing

[soal]

Fixed in version 0.2