[MINOR] Fix the clone commands in the contribution guides

The git access sections told contributors to clone over git://, which
gitbox no longer serves: `git clone git://gitbox.apache.org/repos/asf/zeppelin.git`
fails with a connection refused. The surrounding prose already names the
https URL as the official location, so switch the commands to match it.

The branch example was broken for a second reason: branch-0.11.0 does not
exist on gitbox, so `git clone -b branch-0.11.0` would fail even over
https. The release line is branch-0.11.

Author: kimyenac

docs/development/contribution/how_to_contribute_code.md

@@ -51,13 +51,13 @@ First of all, you need Zeppelin source code. The official location of Zeppelin i
 Get the source code on your development machine using git.
 
 ```bash
-git clone git://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
+git clone https://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
 ```
 
-You may also want to develop against a specific branch. For example, for branch-0.11.0
+You may also want to develop against a specific branch. For example, for branch-0.11
 
 ```bash
-git clone -b branch-0.11.0 git://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
+git clone -b branch-0.11 https://gitbox.apache.org/repos/asf/zeppelin.git zeppelin
 ```
 
 Apache Zeppelin follows [Fork & Pull](https://github.com/sevntu-checkstyle/sevntu.checkstyle/wiki/Development-workflow-with-Git:-Fork,-Branching,-Commits,-and-Pull-Request) as a source control workflow.

docs/development/contribution/how_to_contribute_website.md

@@ -40,7 +40,7 @@ First of all, you need the website source code. The official location of mirror
 Get the source code on your development machine using git.
 
 ```bash
-git clone git://gitbox.apache.org/repos/asf/zeppelin.git
+git clone https://gitbox.apache.org/repos/asf/zeppelin.git
 cd docs
 ```
 Apache Zeppelin follows [Fork & Pull](https://github.com/sevntu-checkstyle/sevntu.checkstyle/wiki/Development-workflow-with-Git:-Fork,-Branching,-Commits,-and-Pull-Request) as a source control workflow.