[Doc] Fix invalid code example in Writing Your Own Field Component tutorial

[Aikain]

Problem

The code example in the documentation is not valid code.

Solution

Corrected example to valid code.

How To Test

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Additional Checks

• The PR targets master for a bugfix, or next for a feature
• The PR includes unit tests (if not possible, describe why) (docs change)
• The PR includes one or several stories (if not possible, describe why) (docs change)
• The documentation is up to date

Also, please make sure to read the contributing guidelines.

[fzaninotto]

Thanks!