Modify git clone command

The previous git clone command is not work, the protocol in URL should be https
  [fix GH-1772]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61199 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

README.md

@@ -41,7 +41,7 @@ command:
 
 Or if you are using git then use the following command:
 
-    $ git clone git://github.com/ruby/ruby.git
+    $ git clone https://github.com/ruby/ruby.git
 
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