Update contains.md

[maurer2]

Hello, it seems that all the other methods of wrapper-array are h2s while contains is an h3 and therefore doesn't show up in the sidebar menu.
Example: https://vue-test-utils.vuejs.org/api/wrapper-array/#exists
Afterwards the deprecation message from wrapper.contains could be added here as well in a subsequent pr.

Cheers

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

• Bugfix
• Feature
• Code style update
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The PR fulfills these requirements:

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[afontcu]

nice catch, thanks!

[maurer2]

Great. Thanks