Yes, there's a command git commit --amend
which is used to "fix" last commit.
In your case it would be called as:
git add the_left_out_file
git commit --amend --no-edit
The --no-edit flag allow to make amendment to commit without changing commit message.
EDIT: Warning You should never amend public commits, that you already pushed to public repository, because what amend does is actually removing from history last commit and creating new commit with combined changes from that commit and new added when amending.