[board] Fix the King's position in the Position::starting()

I wrote a test of the starting position for DiagramFormatter and discovered that Position::starting() was placing the king incorrectly. Fixed it!

board/src/position/diagram_formatter.rs

@@ -58,4 +58,11 @@ mod tests {
         let diagram = DiagramFormatter(&pos);
         println!("{}", diagram);
     }
+
+    #[test]
+    fn starting() {
+        let pos = Position::starting();
+        let diagram = DiagramFormatter(&pos);
+        println!("{}", diagram);
+    }
 }