There is no straightforward way, but there is indeed a way:
- Create a circle object
- Make it editable (Format -> Shape -> Make Editable). This will add the red editing points on the shape.
- To make the editing simpler, rotate the shape 45 degrees (this will put the red editing points at the top and bottom and the sides, rather than arranged as a square)
- Select the object and click on it again to show the edit points, then click on the bottom point to select it and press the Delete key to delete it.
- The resulting shape is close, but the bottom edge is wrong. This is because the angle of the left and right bottom points needs to be adjusted. To do fix this:
* Click on the left point to select it. You will see two grey "handles" project from the point up and down.
* Holding down the Command (Apple) key, click on the circle of the bottom handle, and drag it toward the left point. You are changing the angle of the left edit point in this fashion. Drag the handle so that the handle circle overlays the left edit point
* Repeat the above step for the right edit point.
(You may find that adjusting the angles is more easily done when the circle is larger -- you can always shrink it down later.)
This process may appear cumbersome in description, but it is actually pretty simple to do.