[editorial] Add an alias for tracestate-probability-sampling-experimental (#4691)

- Fixes #4687
- Adds an alias for `tracestate-probability-sampling-experimental` so
that any links to that page into OTel.io, will redirect to
https://opentelemetry.io/docs/specs/otel/trace/tracestate-probability-sampling/

/cc @jmacd @jsuereth

Author: chalin

specification/trace/tracestate-probability-sampling.md

@@ -1,5 +1,6 @@
 <!--- Hugo front matter used to generate the website version of this page:
 linkTitle: Probability Sampling
+aliases: [tracestate-probability-sampling-experimental]
 --->
 
 # TraceState: Probability Sampling