[docs] Fix the appearance of AppVeyor badge
[docs] Fix the appearance of AppVeyor badge

svg version differs from raster version significantly. Only the svg
version looks similar to Travis, Codecov and Codeclimate badges, raster
version doesn't fit.

Change-Id: I45899ea56b776a05c8eb7a71fb55861c018a7c98

wikimedia/pywikibot-core,wikimedia/pywikibot-core,PersianWikipedia/pywikibot-core