insmntque.9: Refer to vnode operations vector correctly

The comment in sys/vnode.h describes
the v_op member as "vnode operations vector",
so use "operations" instead of "operation" here.

Reviewed by: bcr
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D53333

(cherry picked from commit 24a6e040fc2e84234c684abcf3e1668a5a9467eb)