docs README.md

There's a URL mistake when using "git clone" in docs/README.md.

git clone https://gitbox.apache.org/repos/asf/impala.git/docs 

Actually, this doesn't work and it will prompt "repository not found". 

Remark:

On the one hand, if the doc means to clone the whole repository, the command should be

git clone https://gitbox.apache.org/repos/asf/impala.git 

On the other hand, if the doc means to clone only 'docs' directory of the whole repository, "sparse-checkout" could be used and some preceding context may be needed. This is a possible way:

git init impala_docs
cd impala_docs
git remote add origin https://gitbox.apache.org/repos/asf/impala.git
git sparse-checkout set docs/
git pull origin master

You'll see only the 'docs/' sub-directory is downloaded.