fix(Docs): divider in VTreeView example

[J-Sek]

Description

I have stumbled upon a misaligned divider and initially thought it should be rather border-e, but opted to make it play with responsive layout of this example.

Markup

<v-divider vertical />
replaced with:
<v-divider :vertical="$vuetify.display.mdAndUp" class="my-md-3" />

Notes

Technically we could make it default by including .v-divider--vertical in VGrid.sass, but since after migration I have .v-row.my-0 all over my project, I suspect more people might have some margins or paddings that will make this small change introduce unnecessary visual regression... although maybe it is not a strong argument. Feel free to update this PR and include these rules.

.v-row > .v-divider--vertical
  margin-bottom: settings.$grid-gutter * .5
  margin-top: settings.$grid-gutter * .5
  
// and again for `.v-row--dense`