The Guardian style guide guys don't mention the capitalisation of iOS specifically at the beginning of sentences, however they have stated that when the word iPad appears at the start of a sentence, it should be capitalised as Ipad. However, Ios would look strange, and most people (myself included) would probably interpret IOS as a typo.
The Trademarks section of Wikipedia's Manual of Style recommends iOS, but also states that you should try to avoid placing it at the beginning of a sentence.
However, as this is regarding a citation title I don't really think they're actually relevant. I was always taught to cite my sources accurately and not correct errors in the source text where they exist beyond inserting [sic] if necessary, so as the
<h1> tags on the page you linked to are set to
<title>iOS App Distribution Options</title> and
<h1>iOS App Distribution Options</h1> respectively, I would use that capitalisation—iOS App Distribution Options—in my citation.
Incidentally, the block capitalisation of the main header on that site is merely a stylesheet effect which has been applied by the site's designers to the main heading for the page and is not indicative of the capitalisation of the actual text content contained within them.