Fix factor of two typo in weak selection proof

[dandavison]

@cooplab These notes are fantastic, thank you very much for making them available!

I believe I found a typo. I used Wolfram Alpha to check that it gives the solution (eq. 179) to the differential equation. This seems to work in Wolfram Alpha:

solve (s/2)p(1-p)f'(p) + (p(1-p)/(4N))f''(p) = 0, f(1) = 1, f(0) = 0

I created the PDF using latexmk popgen_notes.tex on OS X, but you probably want to remake it on your system to be sure.