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.

puzzles.but

@@ -760,7 +760,7 @@ for, the more likely it is that solutions shorter than the target
 length will turn out to be possible.
 
 
-\C{rectangles} \i{Rectangles}
+\C{rect} \i{Rectangles}
 
 \cfg{winhelp-topic}{games.rectangles}
 

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,