[tutorial] Correct twin font name

We changed the name for the builtin font  from "cairo" to "<cairo>" to
reduce possible naming conflicts - update the tutorial to match.

doc/tutorial/src/twin.c

@@ -20,7 +20,7 @@ draw (cairo_t *cr, int width, int height)
 
     cairo_set_source_rgb (cr, 0, 0, 0);
     cairo_select_font_face (cr,
-			    "cairo",
+			    "<cairo>",
 			    CAIRO_FONT_SLANT_NORMAL,
 			    CAIRO_FONT_WEIGHT_NORMAL);