Add return types to examples

examples/ackermann.sb

@@ -4,7 +4,7 @@ fn main() {
     println(ackermann(m, n))
 }
 
-fn ackermann(m: int, n: int) {
+fn ackermann(m: int, n: int): int {
     if m == 0 {
         return n + 1
     } else if n == 0 {

examples/fib.sb

@@ -3,7 +3,7 @@ fn main() {
     println(fib(num))
 }
 
-fn fib(n: int) { 
+fn fib(n: int): int { 
     if 1 >= n {
         return n
     }

examples/greeter.sb

@@ -2,6 +2,6 @@ fn main() {
     println(greet("World"))
 }
 
-fn greet(name: string) {
+fn greet(name: string): string {
     return "Hello " + name
 }