options: change --volstep to 2

With default settings, this allows you to hit the 100% mark (with
default --softvol-max in the middle) even if you've reached min or max
volume before. This is because 50 is not divisible by 3 (old default)
but by 2 (new default).

Not really sure why there still can be issues with higher --softvol-max
and --volstep=1, but this is where I stop caring.

DOCS/man/options.rst

@@ -975,7 +975,7 @@ Audio
 
 ``--volstep=<0-100>``
     Set the step size of mixer volume changes in percent of the full range
-    (default: 3).
+    (default: 2).
 
 ``--audiodrop``
     If video is by more than 500 ms behind, insert 500 ms of silence, to make up

options/options.c

@@ -558,7 +558,7 @@ const struct MPOpts mp_default_opts = {
     .softvol_max = 200,
     .mixer_init_volume = -1,
     .mixer_init_mute = -1,
-    .volstep = 3,
+    .volstep = 2,
     .gapless_audio = -1,
     .audio_buffer = 0.2,
     .vo = {