Changelog: change "version <next>" to "version 2.3"

Found-by: jamrial
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

Changelog

 Entries are sorted chronologically from oldest to youngest within each release,
 releases are sorted from youngest to oldest.
 
-version <next>:
+version 2.3:
 - AC3 fixed-point decoding
 - shuffleplanes filter
 - subfile protocol