fix(platform): use slash in event handler for apiPrefix

[brandonroberts]

PR Checklist

Please check if your PR fulfills the following requirements:

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

PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Documentation content changes
• Other... Please describe:

Which package are you modifying?

• vite-angular-plugin
• astro-angular
• create-analog
• router
• platform
• content

What is the current behavior?

Issue Number: N/A

What is the new behavior?

Fixes an issue with API routes not being found when deployed on Netlify

Does this PR introduce a breaking change?

• Yes
• No

Other information

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