[manifest] typo found (#342)

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On iOS, when web pages doesn't specify a custom touch icon, the OS takes a screenshot of the webstie and creates an icon from it. This is shown below. 

here
http://w3c-webmob.github.io/installable-webapps/

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Reply to this email directly or view it on GitHub:
https://github.com/w3c/manifest/issues/342