Change git clone links in README.md to a version that does not require special access

[SonicDMG]

Hey all. When following the instructions in the README.md to clone this repo it states to use "git clone [email protected]:datastax/graph-book.git". When I do so I receive the following: Cloning into 'graph-book'... [email protected]: Permission denied (publickey). fatal: Could not read from remote repository.

Please make sure you have the correct access rights and the repository exists.

However, if I simply use the https method provided via the "Clone or Download" button "git clone https://github.com/datastax/graph-book.git" I can clone the repo with no issue.

I am asking this be changed to use the https method so we aren't potentially blocking folks attempting to use the instructions.