Fix documentation link (case-sensitive)

[muan]

This seems to be an issue with swiftpackageindex.com. The lowercase link does not work when linking directly to the page, but it does work when you navigate from their website.

Before, shows "Documentation Error"

$ curl -o - -I https://swiftpackageindex.com/sindresorhus/keyboardshortcuts/main/documentation/keyboardshortcuts/keyboardshortcuts
HTTP/2 404 

https://swiftpackageindex.com/sindresorhus/keyboardshortcuts/main/documentation/keyboardshortcuts/keyboardshortcuts

After

$ curl -o - -I https://swiftpackageindex.com/sindresorhus/KeyboardShortcuts/main/documentation/keyboardshortcuts/keyboardshortcuts
HTTP/2 200 

https://swiftpackageindex.com/sindresorhus/KeyboardShortcuts/main/documentation/keyboardshortcuts/keyboardshortcuts