Fix out-of-date comment on makeRangeConstructors

Hi Hackers,

I noticed that makeRangeConstructors (in commands/typecmds.c) says:

> We actually define 4 functions, with 0 through 3 arguments.

But that hasn't been true since 2011 (df73584431). We only have 2-arg
and 3-arg functions. Here is a quick patch to update the comment.

Yours,

-- 
Paul              ~{:-)
pj@illuminatedcomputing.com