Missing preposition in account signup notes; Unhide refspec mention

Flattened out the contraction in "Establish a GitHub.com Account"
to help improve the overall sentence flow.

Refspec mention on "Git Beyond the Basics" missing the backticks
which hid the chunk from showing up rendered at:

http://teach.github.com/articles/git-intermediate-course/
Merge pull request #184 from randomecho/course-notes-typo

Missing preposition in account signup notes; Unhide refspec mention