Fix bug in get_yaw_rate function.

Thanks to Joan Perez and Sergi Hernandez for spotting it.

src/segway_RMP400.cpp

@@ -207,7 +207,7 @@ CSegwayRMP400::get_yaw_rate()
 {
     need_to_be_connected();
 
-    yaw_rate_ = get_yaw_displacement() * (PI / 180);
+    yaw_rate_ = get_yaw_rate() * (PI / 180);
     return yaw_rate_;
 }