docs: Add a notch to the default square.stl test object

Place a small notch on one side of the square.  The notch provides
interesting information on the extruder performance, and it makes it
easier to compare with previous prints of the object.

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

docs/prints/square.scad

@@ -11,4 +11,6 @@ difference() {
   cube([square_size, square_size, square_height]);
   translate([square_width, square_width, -1])
     cube([square_size-2*square_width, square_size-2*square_width, square_height+2]);
+  translate([-.5, square_size/2 - 4, -1])
+    cube([1, 2, square_height+2]);
 }