fix: update to correct fibonacci example program (#242)

Co-authored-by: John Guibas <[email protected]>

examples/fibonacci/program/src/main.rs

@@ -1,15 +1,19 @@
 #![no_main]
 sp1_zkvm::entrypoint!(main);
 
-pub fn main() {
-    let n = 50000;
-    let mut a = 0;
-    let mut b = 1;
-    let mut sum;
-    for _ in 1..n {
-        sum = a + b;
-        a = b;
-        b = sum;
+use std::hint::black_box;
+
+fn fibonacci(n: u32) -> u32 {
+    let mut nums = vec![1, 1];
+    for _ in 0..n {
+        let mut c = nums[nums.len() - 1] + nums[nums.len() - 2];
+        c %= 7919;
+        nums.push(c);
     }
-    println!("b: {}", b);
+    nums[nums.len() - 1]
+}
+
+pub fn main() {
+    let result = black_box(fibonacci(black_box(50000)));
+    println!("result: {}", result);
 }