doc: fix build

doc: fix build

When removing the label "arm_cross_build_getting_the_prerequisite_library"
in the rework of cross_build_dpdk_for_arm64.rst,
the reference to this section got broken.
It went unnoticed because "ninja -C doc" does not regenerate the file
which is referencing the changed file.

The fix is a reword not mentioning the "not so relevant" example
from cross_build_dpdk_for_arm64.rst.

Fixes: eb0e12c0c299 ("doc: add clang to aarch64 cross build guide")

Signed-off-by: Thomas Monjalon <thomas@monjalon.net>
Reviewed-by: David Marchand <david.marchand@redhat.com>