nppilot: fix a bug in RC scaling.

src/nppilot/scripts/revo.py

@@ -69,7 +69,7 @@ def _scale(x: float, scale: tuple) -> float:
     if x >= 1:
         return high
     if x < 0:
-        return low + (low - zero) * x
+        return zero + (zero - low) * x
     return zero + (high - zero) * x