Add step to documentation to modify `go.mod`

[joshklop]

In #115, the Pulsar dependency was removed (yay!). However, spawn pins the cosmossdk.io/core/appmodule module to v0.11.0, which still requires Pulsar.

We need to add a step to the documentation to manually change the replace directive to use v0.11.1 instead. This step can come in the same section as the other go.mod update (adding a replace for op-geth).