docs: fix typo in upgrade instructions (#8301)
docs: fix typo in upgrade instructions (#8301)

The suggested plugin configuration to re-enable Docker volumes was erroneously
using the singlular `volume` instead of the correct `volumes`, making the
client fail to parse the configuration and causing it not to start.

hashicorp/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,dvusboy/nomad,hashicorp/nomad,hashicorp/nomad,dvusboy/nomad,burdandrei/nomad,hashicorp/nomad,hashicorp/nomad,burdandrei/nomad,hashicorp/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,burdandrei/nomad,dvusboy/nomad,burdandrei/nomad,dvusboy/nomad