dtrace: Improve a verifier error message

Using '%r0' in efunc causes it to parse %r as a 'r' specifier.

This diff just adds a '%' in front of '%r0' in order to create the
correct output.

Reviewed by: markj
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D38176

(cherry picked from commit 7c5bc69a1a33b3a544175ad89c0bba1429491218)