fix docs for AssignableToTypeOf

[ethervoid]

To make the example work we need to include the element to be casted

Fixes #453. in case of a bug fix, this should point to a bug and any other related issue(s)

Description

The example included in the documentation doesn't work. This PR fixes the documentation
to help other people that want to use it to learn how AssignableToTypeOf can be used.

Submitter Checklist

These are the criteria that every PR should meet, please check them off as you
review them:

• Includes tests

Reviewer Notes

• The code flow looks good.
• Tests added.

Release Notes

Describe any changes here so maintainer can include it in the release notes, or delete this block.

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Thanks for your pull request. It looks like this may be your first contribution to a Google open source project (if not, look below for help). Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

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[ethervoid]

@googlebot I signed it!

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CLAs look good, thanks!

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CLAs look good, thanks!

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[codyoss]

Good catch, thank you for your contribution.