doc-src/Tutorial/Recdef/gcd

replaced apply-style proof for instance Multiset :: plus_ac0 by recommended Isar proof style

recdef gcd "measure (%(m,n).n)"
  simpset "simpset() addsimps [mod_less_divisor]"
  "gcd (m, n) = (if n=0 then m else gcd(n, m mod n))"