Typo reported by Greg Stein: "modifiable" is the correct spelling.

Objects/stringobject.c

@@ -479,7 +479,7 @@ string_buffer_getwritebuf(self, index, ptr)
 	const void **ptr;
 {
 	PyErr_SetString(PyExc_TypeError,
-			"Cannot use string as modifyable buffer");
+			"Cannot use string as modifiable buffer");
 	return -1;
 }