docs(guide/Interpolation): fix in markdown (code highlight)

[leonardosouza]

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)
Small fix for code highlight

What is the new behavior (if this is a feature change)?
No

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:

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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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[Narretz]

I know it's a simple change, but even for this we need you to sign the CLA :o.

[leonardosouza]

@Narretz signed!

[googlebot]

CLAs look good, thanks!

[googlebot]

CLAs look good, thanks!