fix #33081: acoustic bass channel info

The difference between 0-based and 1-based patch numbering might be getting in the way here.  I'm just using the same numbering scheme as elsewhere in instruments.xml.  43 is the patch used for the "contrabass" instrument in its default channel, and 32 is the patch number used for the "pizz" channel.  This is 0-based numbering.  In the 1-based numbering you see in most published charts, that's "44 Contrabass" and "33 Acoustic Bass" respectively.  We were already using 32 (=33-1) as the default channel for "Acoustic Bass", and that's correct.  I just needed to add 43 (=44-1) for the arco sound - and 44 (=45-1) for tremolo, so Contrabass and Acoustic Bass have the exact same channels just with a different default.

Now I'm confusing myself, but as far as I can tell, I got it right.  The three patch numbers in question (0-based) are 32 for plucked, 43 for bowed, and 44 for tremolo.  For the "contrabass" instrument, 43 is the default and is unnamed; 32 is available as "pizzicato".  For the "acoustic bass" instrument, 32 is the default and 43 is available as "arco".  This is exactly as it was in 1.3; I'm just restoring that behavior, as for some reason the arco and tremolo options got omitted.