Fix setactivityposition still taking two arguments

[Daaaav]

Changes:

This command was changed from setactivityposition(x,y) to setactivityposition(y) in acca4747f73f066e002a74db5dd2edaacc8ea8d2, but there's a small problem here:

             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.

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• [x] My changes may be used in a future commercial release of VVVVVV
• [x] I will be credited in a CONTRIBUTORS file and the "GitHub Friends" section of the credits for all of said releases, but will NOT be compensated for these changes