libpmc: fix "instructions" alias on Intel

The typo prevents the counter from being allocated.

This fixes e.g. pmcstat -s instructions sleep 5

Reviewed by: mizhka, gnn, ray, emaste
MFC after: 5 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30527

(cherry picked from commit bc1a6a9d692a1f827514144b6bce0654a8be4f4d)