docs: Added comment for handling equal numbers in if/if1.rs `bigger` function

exercises/03_if/if1.rs

@@ -6,6 +6,7 @@
 
 pub fn bigger(a: i32, b: i32) -> i32 {
     // Complete this function to return the bigger number!
+    // If both numbers are equal, any of them is returned.
     // Do not use:
     // - another function call
     // - additional variables