Fix incomplete format error in asdl.py
diff --git a/Parser/asdl.py b/Parser/asdl.py
index 7f5856b..1ddc3f8 100644
--- a/Parser/asdl.py
+++ b/Parser/asdl.py
@@ -126,7 +126,7 @@
         "version ::= Id String"
         if version.value != "version":
             raise ASDLSyntaxError(version.lineno,
-                                msg="expected 'version', found %" % version)
+                                msg="expected 'version', found %s" % version)
         return V
 
     def p_definition_0(self, (definition,)):