[S02] remove unexplained usage of "idfirst"

rir++ asked at http://www.perlmonks.org/?node_id=924669 what "idfirst"
means, and I guess the answer is that simple.

S02-bits.pod

@@ -1831,7 +1831,7 @@ to C<Cool>.
 
 The C<$Package'var> syntax is gone.  Use C<$Package::var> instead.
 (Note, however, that identifiers may now contain an apostrophe or
-hyphen if followed by an "idfirst" letter.)
+hyphen if followed by a character matching C<< <.alpha> >>)
 
 =item *