docs: Fix broken code example for ColorEffect (flame-engine#3382)

ColorEffect in the docs is using an positional parameter for
opacityFrom, but its actually a named parameter.


The docs example for ColorEffect is using a positional parameter instead
of a named parameter, where the definition is using a named parameter.

Closes: flame-engine#3383

Co-authored-by: Jochum van der Ploeg <[email protected]>

doc/flame/effects.md

@@ -538,7 +538,7 @@ Usage example:
 final effect = ColorEffect(
   const Color(0xFF00FF00),
   EffectController(duration: 1.5),
-  opacityFrom = 0.2,
+  opacityFrom: 0.2,
   opacityTo: 0.8,
 );
 ```