[SPARK-4652][DOCS] Add docs about spark-git-repo option

There might be some cases when WIPS spark version need to be run
on EC2 cluster. In order to setup this type of cluster more easily,
add --spark-git-repo option description to ec2 documentation.
"Apache" needs to be capitalized here.  I'd also swap the order of these sentences so that the default for `--spark-git-repo` appears first, followed by the sentence describing how `--spark-version` needs to be set when using this option.