For future reference, you can "square up" any one corner (or more) of a rounded rectangle, as well as altering other shapes. Select the shape, then Format > Shape > Make Editable. You then should see some red dots representing the endpoints of the arc for each rounded corner (in this case). You can reposition them to your heart's content, even bulging them out beyone square for whatever effect. Incidentally, to get back to this mode later, first click to select the shape, then click again to edit it. (NOTE: Not the same as double-clicking, which puts you into text entry mode.)
MacBook Pro Mac OS X (10.4.9)