docs(guide/Unit Testing): add heading

[RonanCodes]

What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)
docs update

What is the current behavior? (You can also link to an open issue here)
Heading missing

What is the new behavior (if this is a feature change)?
Heading present

Does this PR introduce a breaking change?
No

Please check if the PR fulfills these requirements

• The commit message follows our guidelines: https://github.com/angular/angular.js/blob/master/CONTRIBUTING.md#commit-message-format
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

Other information:

Heading missing

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[googlebot]

Thanks for your pull request. It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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Once you've signed, please reply here (e.g. I signed it!) and we'll verify. Thanks.

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[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!

[RonanCodes]

First commit message was incorrect so I added a second one, hopefully I done this correctly.
It is my first time contributing :)