Update iterative_model.rst

line 4   : This lessons shows how to combine  ---> 'lessons' should be 'lesson' (singular)

line 4  :  the iterative execution of algorithm ---> 'algorithm' should be plural 'algorithms'
 
line 30 :  Add the *Slope, aspect, curvatures* algorithm ---->'curvatures' should be singular 'curvature'.