WebFlux reference links to Servlet docs

**Describe the bug**
There's a reference to WebFlux that links to the Servlet documentation:
```
xref:servlet/authentication/index.adoc[WebFlux]
```

**To Reproduce**
See https://docs.spring.io/spring-security/reference/features/authentication/index.html

**Expected behavior**
WebFlux reference should link to reactive section of the documentation.

**Sample**
N/A