List of fixes

1. A Pigeonhole principle: n and k were switched in the statement of the generalised pigeonhole principle.
2. Inclusion-exclusion principle: Page 2, the second line after "Now," had signs reversed for two of the terms.
3. Assignment 4 solutions: In question 4, the equal signs were not properly aligned.
4. Assignment 5 solutions: In question 2d), 36 should be 39.
5. Notes on combinations with repetitions: n+k-1 choose n=n+k-1 choose k-1 should be n+k-1 choose n-1=n+k-1 choose k. The appendix already contained the correct formula.