Minor changes to accessing_a_repo (Pablo)

- Added quick note about how to open terminal on a mac (Applications > Utilities > Terminal), similarly to what you had for a PC.

- Separate the lines of code for 'Substitute your GitHub user information with the user John Doe' so people don't try to run the whole thing at the same time, but rather run each line individually, and then check that it worked with the 'git config --list'.