Rename mapper variable in create_entity() API view

`etype_mapper` is not appropriate since the mapper bound to
ETypeResource is actually a "jsonschema.collection" mapper.

cubicweb_jsonschema/api/entities.py

@@ -129,10 +129,10 @@ def create_entity(context, request):
     # TODO In case of validation errors, it'd be better to give a JSON Schema
     # entry as a "pointer", would require selection context to be an
     # ETypeSchemaResource.
-    etype_mapper = context.mapper(CREATION_ROLE)
+    mapper = context.mapper(CREATION_ROLE)
     instance = request.json_body
-    values = etype_mapper.values(instance)
-    entity = request.cw_cnx.create_entity(etype_mapper.etype, **values)
+    values = mapper.values(instance)
+    entity = request.cw_cnx.create_entity(mapper.etype, **values)
     request.cw_cnx.commit()
     LOG.info('created %s', entity)
     return _created(request, context[entity.eid])