Update CONTRIBUTING.md

[TylerSanford]

Description

Line 44 of CONTRIBUTING.md the command is not highlighting git clone which is a part of the command block.

Also the command git clone [email protected]:<your-username>/gatsby.git doesn't work for me.

Proposed new command line of git clone https://github.com/<your-username>/gatsby.git which does work for me every time.

Steps to reproduce

By viewing the doc CONTRIBUTING.md line 44, also by attempting the command as described: git clone [email protected]:<your-username>/gatsby.git

Expected result

Docs should have the whole command highlighted to eliminate confusion. Also command should properly clone the fork.

Actual result

`[email protected]:~/Desktop$ git clone [email protected]:tylersanford/hello-world.git
Cloning into 'hello-world'...
[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.`

Environment

• Gatsby version (npm list gatsby): N/A - Doc change
• gatsby-cli version (gatsby --version): N/A - Doc change
• Node.js version: node -v: v9.7.1
• Operating System: macOS High Sierra 10.13.4

File contents (if changed):

CONTRIBUTION.md:
* Clone your fork: git clone https://github.com//gatsby.git``