sample-code: Add shebang comments to samples with a main() function

sample-code/fib.cl

@@ -1,3 +1,4 @@
+#!/usr/bin/env -S conlang -r false
 // Calculate Fibonacci numbers
 
 fn main() {

sample-code/fizzbuzz.cl

@@ -1,7 +1,8 @@
+#!/usr/bin/env -S conlang -r false
 // FizzBuzz, using the unstable variadic-`print` builtin
 
 fn main() {
-    fizzbuzz(10, 20)
+    fizzbuzz(0, 30)
 }
 
 // Outputs FizzBuzz for numbers between `start` and `end`, inclusive

sample-code/hello.cl

@@ -1,3 +1,6 @@
+#!/usr/bin/env -S conlang -r false
+//! Prints "Hello, world!"
+
 fn main() {
     println("Hello, world!")
 }