fix: add ': string' to clown-availability's let lastClown (#289)

Author: JoshuaKGoldberg

projects/the-type-system/system-of-a-clown/02-clown-availability/solution.ts

@@ -8,7 +8,7 @@ let ronaldAvailability = true;
 let pennywiseAvailability = true;
 
 let matchingsDescription = "";
-let lastClown;
+let lastClown: string;
 
 do {
 	clownsCount += 1;