DOC: sct_download_data: updated URL to point toward beta28
DOC: sct_download_data: updated URL to point toward beta28

Former-commit-id: 73b9ec6c19e0b3a88181e89913e6b7f244d63fb6 [formerly 2f8b827b429850225018e6f5fb67b7fbf03ed8d2]
Former-commit-id: 9b832cadbc8bae6701c61fe71eb30b100331df3a
Former-commit-id: cfe068e93d316cf9dc67019e9d87acc631e7e082
neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox,neuropoly/spinalcordtoolbox