Missing Vue import

[quriosapien]

import Vue was missing in the code block.
when someone, new to vue tries to follow the guide, it becomes a pain-point to figure why test case which included Vue.nextTick() failed.

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• Other, please describe:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• It's submitted to the dev branch.
• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)
• All tests are passing: https://github.com/vuejs/vue-test-utils/blob/dev/.github/CONTRIBUTING.md#development-setup
• New/updated tests are included

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature (to avoid wasting your time, it's best to open a suggestion issue first and wait for approval before working on it)

Other information:

[lmiller1990]

Looks good, you will probably need to add comments to get this passing.

Eg:

import Vue from 'vue'

// ...
// ...

[quriosapien]

Looks good, you will probably need to add comments to get this passing.

Eg:

import Vue from 'vue'

// ...
// ...

Done. It passed all the tests.

[lmiller1990]

Great, thanks!