Fix SecurityObservationSettings sample in docs

[patbaumgartner]

The "turn on and off observations individually" sample in the Observability
section does not compile against any released version of Spring Security.

Two problems in the same snippet:

• The Java sample calls .shouldObserveFilterChains(true).
  SecurityObservationSettings.Builder exposes shouldObserveRequests,
  shouldObserveAuthentications, shouldObserveAuthorizations and build —
  there is no shouldObserveFilterChains.
• The Kotlin sample additionally calls SecurityObservationSettings.builder().
  The class only offers the static factories noObservations() and
  withDefaults(); there is no builder().

Since the surrounding prose is about re-enabling the filter chain observations
that withDefaults() turns off, shouldObserveRequests(true) is the intended
call — observeRequests is the field that gates
spring.security.http.secured.requests.

This changes documentation only; no runtime behaviour is affected.

I ran into this while checking a training module against the reference docs: the
lab used shouldObserveRequests(true) and I initially "corrected" it to match
the documentation, which then failed to compile.

Happy to fold in the Kotlin builder() fix separately if you would rather keep
the two changes apart.