[doc] Fix example of `Ordering.on` usage.
[doc] Fix example of `Ordering.on` usage.

jvican/scala,felixmulder/scala,lrytz/scala,felixmulder/scala,slothspot/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,lrytz/scala,felixmulder/scala,slothspot/scala,slothspot/scala,jvican/scala,shimib/scala,scala/scala,jvican/scala,felixmulder/scala,jvican/scala,jvican/scala,slothspot/scala,felixmulder/scala,slothspot/scala,slothspot/scala,scala/scala,felixmulder/scala,martijnhoekstra/scala,shimib/scala,shimib/scala,lrytz/scala,martijnhoekstra/scala,shimib/scala,scala/scala,slothspot/scala,jvican/scala,shimib/scala,lrytz/scala,shimib/scala,scala/scala,martijnhoekstra/scala,lrytz/scala,martijnhoekstra/scala,scala/scala,martijnhoekstra/scala,felixmulder/scala,jvican/scala