Fix typo in Chapter 5, section "Public Large Project", in `format-patch`
example

Fixes issue github/gitscm-next#313

Chapter 5 contains a typo (two hyphens instead of the correct three
hyphens) in an example for 'format-patch' in the Public Large Project
section.
Merge pull request #533 from jrstjohn/fix-issue-313

Fix typo in Chapter 5, section "Public Large Project", in `format-patch` example