Fix wrong term in battery formula documentation

Signed-off-by: Mathias L. Baumann <[email protected]>

Author: Marenz

src/frequenz/sdk/timeseries/formula_engine/_formula_generators/_battery_power_formula.py

@@ -24,7 +24,7 @@
 
 
 class BatteryPowerFormula(FormulaGenerator[Power]):
-    """Creates a formula engine from the component graph for calculating grid power."""
+    """Creates a formula engine from the component graph for calculating battery power."""
 
     def generate(
         self,