Update custom_events.md

[ipavan]

fix to the code view, changing a semi-colon to a colon

Pull Request/Issue Resolution

Description of Change:

Fixing one bit in the code view, changing a semi-colon to a colon

Closes #ISSUE_NUMBER_HERE

Is this change associated with a Braze feature/product release?

• Yes (Insert Feature Release Date Here)
• No

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[ipavan]

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