Spring Security Kotlin DSL examples in reactive oauth2 docs call build twice

This issue addresses incorrect usage of the Kotlin DSL related to documentation improvements under gh-8174.

    @Bean
    fun securityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
        http {
            ...
        }

        return http.build()
    }

Should be:

    @Bean
    fun securityFilterChain(http: ServerHttpSecurity): SecurityWebFilterChain {
        return http {
            ...
        }
    }