What you are seeing is a tab. It is part of the Style of your header; therefore, you must modify the style, or create a new one, to eliminate the tab. I don't know the Mac, but I believe you find Styles listed under Format in the menu bar in your version of Word. That should open up a pane at the right side of your screen with a list of styles. Right click the relevant heading style, select modify, and then click on Format at the bottom of the dialogue box that comes up.