Changed mirror paths in this guide to remove hard-coded references to
stx-r1, pike, and cgcs-centos.
This edit is a follow-on to Build Guide review:
https://review.opendev.org/#/c/757932/
Similar changes to the Layered Build Guide & Ref (English) are in:
https://review.opendev.org/#/c/762761/
Change-Id: I904423cbdca152b1fd31c3b7143f641d23182e66
Signed-off-by: MCamp859 <maryx.camp@intel.com>