docs: update the readme

Author: KazuCocoa

README.md

@@ -10,7 +10,7 @@ This library wraps [selenium-webdriver](https://github.com/SeleniumHQ/selenium/w
 
 # Documentation
 
-- http://www.rubydoc.info/github/appium/ruby_lib_core
+- https://www.rubydoc.info/gems/appium_lib_core
     - You can find working API examples in test code, [test/functional](test/functional)
 
 # Migration from v4 to v5