Doc fix: dynamic options

When the value of a `dynamic` option is updated core Lightning
will make a JSON-RPC request to the plugin.

The documentation falsely mentioned the request has a method named
`setvalue. It appears it is actually called `setconfig`.

doc/developers-guide/plugin-development/a-day-in-the-life-of-a-plugin.md

@@ -89,7 +89,7 @@ The `getmanifest` method is required for all plugins and will be called on start
 }
 ```
 
-During startup the `options` will be added to the list of command line options that `lightningd` accepts. If any `options` "name" is already taken startup will abort. The above will add a `--greeting` option with a default value of `World` and the specified description. _Notice that currently string, integers, bool, and flag options are supported._ If an option specifies `dynamic`: `true`, then it should allow a `setvalue` call for that option after initialization.
+During startup the `options` will be added to the list of command line options that `lightningd` accepts. If any `options` "name" is already taken startup will abort. The above will add a `--greeting` option with a default value of `World` and the specified description. _Notice that currently string, integers, bool, and flag options are supported._ If an option specifies `dynamic`: `true`, then it should allow a `setconfig` call for that option after initialization.
 
 The `rpcmethods` are methods that will be exposed via `lightningd`'s JSON-RPC over Unix-Socket interface, just like the builtin commands. Any parameters given to the JSON-RPC calls will be passed through verbatim. Notice that the `name`, `description` and `usage` fields  
 are mandatory, while the `long_description` can be omitted (it'll be set to `description` if it was not provided). `usage` should surround optional parameter names in `[]`.