Fix sizes property name to size in README

[PetrDlouhy]

Fix #38 The sizes property name in README doesn't match with the code, where is it named size: https://github.com/silviolleite/django-pwa/blob/0ff60e7afaf23d6883d593554b93e54bb56e41fe/pwa/templates/pwa.html#L19

[JelleBootsma]

I believe this should be fixed in pwa.html, instead of in the readme. The icon property should be called sizes, as this also injected into the manifest.json and here it should be sizes, as can be seen on MDN Web Docs