doc-src/Tutorial/Recdef/ack

fixed two obscurities of "fix": predeclare_terms;

consts ack :: "nat*nat => nat"
recdef ack "measure(%m. m) <*lex*> measure(%n. n)"
  "ack(0,n)         = Suc n"
  "ack(Suc m,0)     = ack(m, 1)"
  "ack(Suc m,Suc n) = ack(m,ack(Suc m,n))"