Rename the docs section for Rectangles.

The web page currently assumes it's called 'rect' rather than
'rectangles', because the web-page building script uses the first
field of each line of gamedesc.txt, same as the Unix binary name.
Rather than add another confusingly-almost-identical field to that
file, it's easier to just rename this one docs section to make the
assumption of equality hold.

rect.c

@@ -2960,7 +2960,7 @@ static void game_print(drawing *dr, const game_state *state, int tilesize)
 #endif
 
 const struct game thegame = {
-    "Rectangles", "games.rectangles", "rectangles",
+    "Rectangles", "games.rectangles", "rect",
     default_params,
     game_fetch_preset,
     decode_params,