Fixed versioning regexp

booking list regexp was fixed in 8b97bb8f68 but single booking wasn't for some reason, and `v1v2/bookings/123/` shouldn't be allowed here

docs/api-guide/versioning.md

@@ -135,7 +135,7 @@ Your URL conf must include a pattern that matches the version with a `'version'`
             name='bookings-list'
         ),
         url(
-            r'^(?P<version>[v1|v2]+)/bookings/(?P<pk>[0-9]+)/$',
+            r'^(?P<version>(v1|v2))/bookings/(?P<pk>[0-9]+)/$',
             bookings_detail,
             name='bookings-detail'
         )