Fix a typo with modchart docs

I swear to god this one typo had me pulling my hair out for like 15 minutes trying to troubleshoot, telling myself "The Sample Code HAS to be right". It wasn't, plz fix.

ModCharts.md

@@ -70,7 +70,7 @@ function update (elapsed)
 	else
         for i=0,7 do
             setActorX(_G['defaultStrum'..i..'X'],i)
-            setActorX(_G['defaultStrum'..i..'Y'],i)
+            setActorY(_G['defaultStrum'..i..'Y'],i)
         end
     end
 end