doc/filters: fix copy&paste error in volume_justin documentation

Spotted by ubitux.

doc/filters.texi

@@ -927,7 +927,7 @@ The filter accepts the following named parameters. If the key of the
 first options is omitted, the arguments are interpreted according to
 the following syntax:
 @example
-drawbox=@var{volume}:@var{precision}
+volume=@var{volume}:@var{precision}
 @end example
 
 @table @option