Update doc that "-retime" calls abc with "-dff -D 1"

techlibs/ice40/synth_ice40.cc

@@ -65,7 +65,7 @@ struct SynthIce40Pass : public ScriptPass
 		log("        do not flatten design before synthesis\n");
 		log("\n");
 		log("    -retime\n");
-		log("        run 'abc' with -dff option\n");
+		log("        run 'abc' with '-dff -D 1' options\n");
 		log("\n");
 		log("    -nocarry\n");
 		log("        do not use SB_CARRY cells in output netlist\n");