doc: update quickstart guide syntax highlighting

Use shell syntax highlighting where appropriate in the Quickstart guide
for consistent syntax highlighting across our documentation.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Yefeng Lin <yefeng.lin@arm.com>
Reviewed-by: Liangxing Wang <liangxing.wang@arm.com>

Change-Id: Iecaaffb397cbd2e5d3db4d0d3e41c284f6c7b54e