Update 6-viewsets-and-routers.md

[dmitrijBerg]

Motivation

Following part 6 of the tutorial, I encountered following error:

Could not resolve URL for hyperlinked relationship using view name "user-detail". You may have failed to include the related model in your API, or incorrectly configured the `lookup_field` attribute on this field.

After some debugging I tracked the problem down to two lines of code at the very end.

Description

The basename values users and snippets break the automatic creation of urlpatterns. Instead, it should be user and snippet.

[lovelydinosaur]

Resolved in #8517

(And ooops, yes I see that this one came first, thanks anyway)