doc: update build command in BlueField guide
doc: update build command in BlueField guide

Since linuxapp in build configs has been changed to linux, change it
in the bluefield doc.

Fixes: 22d1d1ccc357 ("doc: add Mellanox BlueField platform guide")
Cc: stable@dpdk.org

Signed-off-by: Yong Wang <d43454047dd416473224614364813c3ab52b395d@zte.com.cn>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk