Fix download procedure at IC doc to use quillback branch

Fix download procedure at IC doc to use quillback branch

The  "### 6th step: Download the AGL Source Files"  has the description
of repo init command.  But it is not use quillback branch.
When developer use this command, repo download master branch.

This patch add branch setting into the description of repo init command.

Bug-AGL: SPEC-5224

Change-Id: I7c81cfefd659cf4f2be9f118cfacdeb157f884ee
Signed-off-by: Naoto Yamaguchi <naoto.yamaguchi@aisin.co.jp>
Reviewed-on: https://gerrit.automotivelinux.org/gerrit/c/AGL/documentation/+/30162
Tested-by: Walt Miner <walt@linux.com>
Reviewed-by: Walt Miner <walt@linux.com>