Docs reference a `putChannel` mutation but none such exists

https://diamondlightsource.github.io/coniql/main/how-to/run-a-server.html#ca-plugin says that one may put to a pv using a query of the format:

```graphql
mutation {
putChannel(id: "ca://pc0105:longout", value: "45") {
value {
string(units: true)
}
}
}
```

however, the only mutation which exists is `putChannels` which takes a list of PVs and values