Create Your First Package tutorial fixes

When following this tutorial, I had some problems in the "Add a Key Binding" section:

1. It told me to put `atom-text-editor` on the first line, which does not work. However, using `.workspace` works, so I have changed the tutorial to say this instead.

2. It told me in a paragraph to enter `ctrl-alt-a`, but the example text read `cmd-alt-a` instead. Thus, I changed the `cmd` to `ctrl`.

3. It told me that deleting the existing key binding will leave me with a two-line file, which is not true because of the default comments at the top of the file. I therefore changed the wording slightly.