Adds Context to attachTo Option Example

[CentroDL]

Previously, the documentation for attachTo provided an example where a component was being attached to a div with an id of root, but without it being defined it's kind of distracting. This update explicitly defines the element and attaches it to the document for clarity.

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

• Bugfix
• Feature
• Code style update
• Refactor
• Build-related changes
• [ x] Other, please describe: Documentation Change

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

• Yes
• [ x] No

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

The PR fulfills these requirements:

• [ x] 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)
• [ x] 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: