Update HACKING.md to refer to Dune instead of JBuilder
Update HACKING.md to refer to Dune instead of JBuilder

A handful of `jbuild` references remain, reflecting source code file names.

Signed-off-by: Shon Feder <ea1e9e869ba967b9537eb0b16fe47f610d2cc997@gmail.com>

dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder,janestreet/jbuilder,janestreet/jbuilder,dra27/jbuilder,dra27/jbuilder,janestreet/jbuilder