Setting header content-type to text/plain will tell the browser that the file is a text file, hence the HTML markup will be shown as text too.
I would guess that the html is not supposed to be in the file at all, so just remove it and it will look like you want it to.
Other than that, headers are supposed to be set before any type of output, so set them at the beginning of the file, before any output.
<?php
header("Content-Type: text/plain");
echo("Hello");