Update producer references in documentation

Signed-off-by: Daniel Zullo <[email protected]>

Author: daniel-zullo-frequenz

src/frequenz/sdk/timeseries/formula_engine/_formula_engine.py

@@ -332,7 +332,7 @@ def from_receiver(
             from frequenz.sdk.timeseries import Power
 
             async def run() -> None:
-                producer_power_engine = microgrid.logical_meter().producer_power
+                producer_power_engine = microgrid.producer().power
                 consumer_power_recv = microgrid.consumer().power.new_receiver()
 
                 excess_power_recv = (