Fix incorrect URL to Swagger UI in getting-started

[toresbe]

I'm an absolute newbie in the Kotlin world so it could just as well be that I'm doing something odd here, but following this guide, I discovered that the file was served at swagger-ui/index.html and not swagger-ui.html.

[bnasslahsen]

@toresbe,

Thank you for your contribution to the project You should only update the .adoc files. The files are located here: https://github.com/springdoc/springdoc.github.io/tree/master/src/docs/asciidoc/