Fix: update docs about snd

docs/forth/control_flow.md

@@ -112,7 +112,7 @@ Reads naturally: "c3 or c5, depending on the coin."
 
 ```forth
 ( 0.8 gain ) ( 0.3 gain ) fill ifelse
-tri s c4 note 0.2 decay .
+tri snd c4 note 0.2 decay .
 ```
 
 Loud during fills, quiet otherwise.
@@ -123,7 +123,7 @@ Choose the nth quotation from a list. The index is 0-based:
 
 ```forth
 ( c4 ) ( e4 ) ( g4 ) ( b4 ) 0 3 rand select
-note sine s 0.5 decay .
+note sine snd 0.5 decay .
 ```
 
 Four notes of a major seventh chord picked randomly. Note that this is unnecessarily complex :)