FAQ links to wrong file for OpenApiCustomiser example

[aubreyyan]

Is your feature request related to a problem? Please describe.

• A clear and concise description of what the problem is. Ex. I'm always frustrated when [...]

I'm always frustrated when I click on a link from documentation and it's an empty test file.

• What is the actual result using OpenAPI Description (yml or json)?

Describe the solution you'd like

• A clear and concise description of what you want to happen.
• What is the expected result using OpenAPI Description (yml or json)?

Update link to https://github.com/springdoc/springdoc-openapi/blob/master/springdoc-openapi-webflux-core/src/test/java/test/org/springdoc/api/app39/SpringDocTestApp.java

Describe alternatives you've considered

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None

Additional context

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None