Minor correction to My First Transaction docs

Under the “My First Transaction” docs (https://developers.libra.org/docs/my-first-transaction) the instructions are to “Clone the Libra Repository” and then “Checkout the Test Branch”. However, you need to change to the libra directory using “cd libra” before running the “git checkout testnet” command otherwise it will not work. A very minor correction needed to the docs, but I missed this and spent 50 minutes waiting for it to build something that then didn’t work!!

If it’s helpful to another people, I think you can contribute your solution into github. Make sure no one already created the pull request. Ref: https://github.com/libra/website

But look like it is simple for the developer. I’m not sure reviewer will response.