Fix setactivityposition still taking two arguments

This command was changed from setactivityposition(x,y) to
setactivityposition(y), but there's a small problem here:

```diff
             else if (words[0] == "setactivityposition")
             {
-                obj.customactivitypositionx = ss_toi(words[1]);
                 obj.customactivitypositiony = ss_toi(words[2]);
             }
```

This meant that the function still took two arguments, the first of
which was unused and the second of which was the Y position of the
activity zone. This is now fixed.

desktop_version/src/Script.cpp

@@ -1683,7 +1683,7 @@ void scriptclass::run(void)
             }
             else if (words[0] == "setactivityposition")
             {
-                obj.customactivitypositiony = ss_toi(words[2]);
+                obj.customactivitypositiony = ss_toi(words[1]);
             }
             else if (words[0] == "createrescuedcrew")
             {