Update custom HealthIndicator docs

The [custom health indicator documentation](http://docs.spring.io/spring-boot/docs/1.2.0.RELEASE/reference/htmlsingle/#_writing_custom_healthindicators) shows that it should be possible to simply return ```Health.up()```, but this is no longer valid since the introduction of the new ```Health.builder``` changes with 057e149

This pull request updates the documentation to use the builder rather than the old mechanism.

I've signed the CLA