mixer.8: Fix wrong sentence

246e0457d9 ("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

usr.sbin/mixer/mixer.8

@@ -223,11 +223,11 @@ List all available audio devices in the system:
 $ mixer -a | grep ^pcm
 .Ed
 .Pp
-Increase the volume for the
+Increase the
 .Cm vol
-device of the first mixer found by 5%:
+device's volume by 5%:
 .Bd -literal -offset indent
-$ mixer vol=+0.05
+$ mixer vol=+5%
 .Ed
 .Pp
 Change the volume for the