Fix incomplete format error in asdl.py
diff --git a/Parser/asdl_c.py b/Parser/asdl_c.py
index 3d46220..44c3a6d 100755
--- a/Parser/asdl_c.py
+++ b/Parser/asdl_c.py
@@ -1,4 +1,4 @@
-#! /usr/bin/env python
+#! /usr/bin/env python2
 """Generate C code from an ASDL description."""
 
 # TO DO