fix table markdown

Author: andrewconnell

docs/design/semantic_slots.md

@@ -29,7 +29,8 @@ In the table below, you can see all eight palette colors defined for the bodyTe
 
 ![Example table showing semantic slots on light and dark theme variants](../images/doc-semantic-slot-940px-table.png)
 
-|         | Light themes           | Dark themes            |
+|         |      Light themes      |      Dark themes       |
+| ------- | ---------------------- | ---------------------- |
 | None    | neutralPrimary #333333 | neutralPrimary #ffffff |
 | Neutral | neutralPrimary #333333 | neutralPrimary #ffffff |
 | Soft    | neutralPrimary #333333 | neutralPrimary #ffffff |