Corrected example which still used old primrec syntax.

--- a/doc-src/Ref/simplifier.tex	Tue Jul 25 17:47:55 2000 +0200
+++ b/doc-src/Ref/simplifier.tex	Tue Jul 25 18:43:52 2000 +0200
@@ -1091,7 +1091,7 @@
 \begin{ttbox}
 NatSum = Arith +
 consts sum     :: [nat=>nat, nat] => nat
-primrec "sum" nat 
+primrec 
   "sum f 0 = 0"
   "sum f (Suc n) = f(n) + sum f n"
 end