docs: fix links & headings about built-in samplers (#2011)
docs: fix links & headings about built-in samplers (#2011)

Co-authored-by: Valentin Marchaud <1a73af9e7ae00182733b2292511b814be66f065f@vmarchaud.fr>
open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js,open-telemetry/opentelemetry-js