Typo reported by Greg Stein: "modifiable" is the correct spelling.
diff --git a/Objects/stringobject.c b/Objects/stringobject.c
index dffc8e0..648e6e2 100644
--- a/Objects/stringobject.c
+++ b/Objects/stringobject.c
@@ -479,7 +479,7 @@
 	const void **ptr;
 {
 	PyErr_SetString(PyExc_TypeError,
-			"Cannot use string as modifyable buffer");
+			"Cannot use string as modifiable buffer");
 	return -1;
 }