Fix slack link (#1403)

README.md

@@ -13,7 +13,7 @@
 ![core gzip size](https://img.shields.io/bundlephobia/minzip/emotion.svg?label=core%20gzip%20size)
 ![react size](https://img.shields.io/bundlephobia/min/@emotion/styled-base.svg?label=react%20size)
 ![react gzip size](https://img.shields.io/bundlephobia/minzip/@emotion/styled-base.svg?label=react%20gzip%20size)
-[![slack](https://emotion-slack.now.sh/badge.svg)](https://emotion-slack.now.sh//)
+[![slack](https://emotion-slack.now.sh/badge.svg)](https://emotion-slack.now.sh/)
 [![spectrum](https://withspectrum.github.io/badge/badge.svg)](https://spectrum.chat/emotion)
 
 Emotion is a performant and flexible CSS-in-JS library. Building on many other CSS-in-JS libraries, it allows you to style apps quickly with string or object styles. It has predictable composition to avoid specificity issues with CSS. With source maps and labels, Emotion has a great developer experience and great performance with heavy caching in production.