Yes, there’s a command, git commit --amend, which is used to “fix” the 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 allows to make an amendment to the commit without changing the commit message.
Warning
You should never amend public commits that you already pushed to a public repository, because amend is actually removing the last commit from the history and creating a new commit with the combined changes from that commit and new added when amending.