write_riemann plugin: Fix two bugs in the example configuration.

Remove a trailing ">" from the example config and use the correct config
item to configure the "StoreRates" option.
write_riemann plugin: Update copyright information.
write_riemann plugin: Use format_name() to generate the service name.

This uses the default "plugin-instance/type-instance" format, optionally
adding the ds name if the type has more than one data source.