docs/manual: fix Config.in option that cargo packages must depend on

docs/manual: fix Config.in option that cargo packages must depend on

Back in commit 025b863e6facb7b219b82ee6ee7a7916c2c3c47e, the option
BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS was introduced, to
separate the option that host packages needing Rust should depend on
(BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS) from the option that target
packages needing Rust should depend on
(BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS).

Since the example in the manual is showing a target package, we must
use BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Sam Voss <sam.voss@gmail.com>
Cc: Eric Le Bihan <eric.le.bihan.dev@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>