Update to new SVN URL in readme.txt

Signed-off-by: Tian, Hot <hot.tian@intel.com>
Reviewed-by: Ni, Ruiyu <ruiyu.ni@intel.com>
Reviewed-by: Gao, Liming <liming.gao@intel.com>


git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@14399 6f19259b-4bc3-4df7-8a09-765794883524