`docs/user/configuration/traces-config.md` contains wrong information about configuring `tail_sampling.policies`

[bartoszbehring-form3]

Recently we wanted to configure tail sampling policies the agent. We followed the documentation that mentions you should define them in the same way as it is in the Open Telemetry Collector. For example:

tail_sampling:
  policies:
     [
          {
            name: test-policy-1,
            type: always_sample
          }
     ]

It turned out with such configuration the agent was returning following error. After digging out in the Grafana Agent code we figured it out that, we should do it in the following way instead:

tail_sampling:
  policies:
    - always_sample: ""

However I'm not sure if it's just a documentation being wrong or it is the bug in the code and the Grafana Agent would like to follow the Open Telemetry Collector scheme for this field.