to_spin_string: fix output of false and true

We used to output 0 and 1, but "spin -f" does not under understand
that.

* src/ltlvisit/tostring.cc (kw_spin): Output true and false instead of
0 and 1.

src/ltlvisit/tostring.cc

@@ -99,8 +99,8 @@ namespace spot
       };
 
       const char* spin_kw[] = {
-	"0",
-	"1",
+	"false", // 0 doesn't work from the command line
+	"true",  // 1 doesn't work from the command line
 	"[*0]",			// not supported
 	" xor ",		// rewritten
 	" -> ",			// rewritten, although supported