revlog: use %d to format int instead of %lu (issue6565)

The issue says gcc warns that the data types don't match. I couldn't reproduce
the warning locally for some reason, but this patch shouldn't break things.
Maybe %lu was simply a copy-paste error from 6b1eae313b2f
(https://phab.mercurial-scm.org/D10625).

mercurial/cext/revlog.c

@@ -497,7 +497,7 b' static PyObject *index_pack_header(index'
 	if (self->format_version != format_v1) {
 		PyErr_Format(PyExc_RuntimeError,
 		             "version header should go in the docket, not the "
-		             "index: %lu",
+		             "index: %d",
 		             header);
 		return NULL;
 	}