docs(readme): update nightly dependency url

As of the github disabling on support of 'git://' protocol,
update nightly dependency url

https://github.blog/2021-09-01-improving-git-protocol-security-github/#no-more-unauthenticated-git
https://docs.npmjs.com/cli/v8/configuring-npm/package-json#github-urls

README.md

@@ -115,7 +115,7 @@ There're two ways to install from `nightly` branch directly.
 // Specify on 'package.json' file
 "dependencies": {
       ...
-      "billboard.js": "git://github.com/naver/billboard.js.git#nightly"
+      "billboard.js": "naver/billboard.js#nightly"
 },
 ```