Add teaching note using gedit at git lesson

with suggestions by @wking.

Close #455.
Small fix to notes about Gedit

Thanks to @gavinsimpson.
Merge branch 'git-editor' of github.com:r-gaia-cs/bc into r-gaia-cs-git-editor
Editing language of note on Gedit for git commits
Merge pull request #478 from gvwilson/r-gaia-cs-git-editor

Editing language of #456 (note on Gedit for git commits)