docs: fix GitHub format in README (#3335)

Github -> GitHub

Author: Ikko Ashimine

README.md

@@ -246,7 +246,7 @@ Support us with a monthly donation and help us continue our activities. [[Become
 <a href="https://opencollective.com/pug/backer/29/website" target="_blank"><img src="https://opencollective.com/pug/backer/29/avatar.svg"></a>
 
 ## Sponsors
-Become a sponsor and get your logo on our README on Github with a link to your site. [[Become a sponsor](https://opencollective.com/pug#sponsor)]
+Become a sponsor and get your logo on our README on GitHub with a link to your site. [[Become a sponsor](https://opencollective.com/pug#sponsor)]
 
 <a href="https://opencollective.com/pug/sponsor/0/website" target="_blank"><img src="https://opencollective.com/pug/sponsor/0/avatar.svg"></a>
 <a href="https://opencollective.com/pug/sponsor/1/website" target="_blank"><img src="https://opencollective.com/pug/sponsor/1/avatar.svg"></a>