Sustituímos i=1 por i=0
<meta charset="utf-8">
<h1>Lista Heroes</h1>
<script>
function saltarLinea(){
document.write("<br>");
}
function imprimir(frase){
document.write(frase);
saltarLinea()
}
var heroes = ["Superman", "Thor", "Batman", "Mujer Maravilla"];
for( var i = 0; i < heroes.length; i++) {
imprimir(heroes[i]);
}
</script>