Spelling correction on read_only_fields err msg

just spotted extras

rest_framework/serializers.py

@@ -685,10 +685,10 @@ class ModelSerializer(Serializer):
             assert field_name not in self.base_fields.keys(), \
                 "field '%s' on serializer '%s' specfied in " \
                 "`read_only_fields`, but also added " \
-                "as an explict field.  Remove it from `read_only_fields`." % \
+                "as an explicit field.  Remove it from `read_only_fields`." % \
                 (field_name, self.__class__.__name__)
             assert field_name in ret, \
-                "Noexistant field '%s' specified in `read_only_fields` " \
+                "Non-existant field '%s' specified in `read_only_fields` " \
                 "on serializer '%s'." % \
                 (field_name, self.__class__.__name__)
             ret[field_name].read_only = True