Fix typo.

models/tshock.mod

@@ -86,7 +86,7 @@ shocks;
 
 end;
 
-perfect_foresight_setup(periods=@{PERIODS_VALUE});
+perfect_foresight_setup(periods=@{PERIODS});
 perfect_foresight_solver(no_homotopy, markowitz=@{MARKOWITZ_VALUE}, stack_solve_algo = @{STACK_SOLVE_ALGO_VALUE}, solve_algo = @{STEADY_SOLVE_ALGO_VALUE});
 
 end;