improve/fix node performance section

[rampall]

https://docs.ethswarm.org/docs/bee/working-with-bee/staking#check-node-performance

1. replace randomstring with <random-hex-string>
2. update rchash output form - it looks very different from the one shown in the docs
3. looks like it has to be a minimum length? curl -X GET http://localhost:3051/rchash/8/aaa returns invalid hex params error