Merge pull request progit#209 from MasoodFallahpoor/ch06-typo-fix

Fix a typo in "contributing" section of chapter 6

Author: jnavila

book/06-github/sections/2-contributing.asc

@@ -252,7 +252,7 @@ If you absolutely wish to rebase the branch to clean it up, you can certainly do
 
 ===== References
 
-Your next question may be ``How to I reference the old Pull Request?''. It turns out there are many, many ways to reference other things almost anywhere you can write in GitHub.
+Your next question may be ``How do I reference the old Pull Request?''. It turns out there are many, many ways to reference other things almost anywhere you can write in GitHub.
 
 Let's start with how to cross-reference another Pull Request or an Issue. All Pull Requests and Issues are assigned numbers and they are unique within the project. For example, you can't have Pull Request #3 _and_ Issue #3. If you want to reference any Pull Request or Issue from any other one, you can simply put `#<num>` in any comment or description. You can also be more specific if the Issue or Pull request lives somewhere else; write `username#<num>` if you're referring to an Issue or Pull Request in a fork of the repository you're in, or `username/repo#<num>` to reference something in another repository.