doc badge links to https (#480) [skipci]

README.md

@@ -15,10 +15,10 @@ You can use ColorTypes as a standalone package if you do not need the more exten
 Basic color arithmetic is available with `using ColorVectorSpace`; see [its documentation](https://github.com/JuliaGraphics/ColorVectorSpace.jl) for why it's a separate package.
 
 [docs-dev-img]: https://img.shields.io/badge/docs-dev-blue.svg
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 [docs-stable-img]: https://img.shields.io/badge/docs-stable-blue.svg
-[docs-stable-url]: http://juliagraphics.github.io/Colors.jl/stable/
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