It happens that you add changes to the staging area, then you realize they fit better in a future commit, not in the commit you are composing right now.
To remove those changes to one or more files from the staging area, you can use the git reset HEAD <file> command; get back the shell and follow me.
Check the repository current status:
 ~/grocery (master) $ git status On branch master Changes to be committed: (use "git reset HEAD <file>..." to unstage) modified: shoppingList.txt Changes not staged for commit: (use "git add <file>..." to update what will be committed) (use "git checkout -- <file>..." to discard changes in working directory) modified: shoppingList.txt
This is the actual situation, ...