[docs] mark transactionSampleRate as dynamic

[toby-sutor]

If I am not mistaken, should the transactionSampleRate be marked as Dynamic in the documentation as it is the case for example on the Java Agent.

Based on the Kibana documentation:

Supported configurations are also tagged with the dynamic config badge in each Agent’s configuration reference

And from the RUM documentation:

Currently, only transaction sample rate can be configured via Kibana.