Added description of bug fix to changelog

CHANGES/v2.3.txt

@@ -158,6 +158,7 @@ See branch \branch{v2.3} on git repository.
 - Fixed bug in \ref HBONDS that used to apear when you used DONORNS and ACCEPTORS with same numbers of atoms 
 - Fixed bug in \ref DISTANCES that appears when using BETWEEN and link cells.
 - Prevented users from causing segfaults by storing derivatives without LOWMEM flag.  In these caess PLUMED crashes with meaningful errors.
+- Fixed bug in \ref HISTOGRAM that causes nans when using KERNEL=DISCRETE option
 
 For developers:
 - plumedcheck validation has been made stricter. All the checks are now described in the developer manual.