Update README.md
macOS has been the official name since 2016. Previously the name was OS X (2012 to 2016) and again before Mac OS X (2001 to 2012). (→ https://en.wikipedia.org/wiki/MacOS) + draw attention to an interesting feature (arrows at the end of the rulers)
Also useful for search results on GitHub with About: A ruler application for Mac OS X → A ruler application for macOS
Thanks for this PR, sorry it took me so long to look at it. Decided to just use "Mac OS" even though it's not on-brand because I find the lowercaseUPPERCASE branding kinda silly, and I don't work for Apple so I don't have to match their guidance here. But I recognize that "Mac OS X" is pretty outdated at this point so I removed the X : )