doc: fix DESTDIR variable name in meson guide
doc: fix DESTDIR variable name in meson guide

ninja does not recognize 'DEST_DIR'.

Fixes: 9c3adc289c5e ("doc: add instructions on build using meson")
Cc: stable@dpdk.org

Signed-off-by: Ilya Maximets <e6f59449982e74b9a8013e88011af79c397f66d7@samsung.com>
Acked-by: Bruce Richardson <66b9e8e8fbd5e9f41f91b0af050876182ce9d0e4@intel.com>

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