Fix formatting of `@Bean` in reference documentation

[sahrawat]

Quoting https://docs.spring.io/spring-boot/docs/2.3.1.RELEASE/reference/htmlsingle/#production-ready-metrics-web-flux

To add to the default tags, provide one or more @Bean`s that implement `WebFluxTagsContributor

I suppose it's a encoding problem.

[snicoll]

@sahrawat good catch, thank you for reporting it.