mixer.8: Fix wrong sentence

246e0457d93071ffd901c78e3ee7badc5f51bd4c ("mixer.8: Add terse example
for increasing volume") mentions that the example changes the volume of
the "first mixer found", while the example shows how the change the
volume of the current mixer's "vol" device. Re-phrease sentence to
reflect the actual behavior of the command.

Also, improve the example by using the % operator, instead of hardcoding
0.05.

Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks
Reviewed by: dev_submerge.ch, markj
Differential Revision: https://reviews.freebsd.org/D43795