Corrections

In the Network Flows and Comb. Matrix Theory notes there are some corrections (updated file is on the web page): (1) Page 9, line 9 from top: u should be c and l should be 0, (2) Page 20, Exercise 2: typo in the sums, (3) Page 14, Thm 10, ... S be nonincreasing, nonnegative integral ..., (4) Page 13, proof of Thm 2.3: b(v_j)=-s_j (the minus was left out)