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>

docs/manual/adding-packages-cargo.txt

@@ -17,7 +17,7 @@ The +Config.in+ file of Cargo-based package 'foo' should contain:
 ---------------------------
 01: config BR2_PACKAGE_FOO
 02: 	bool "foo"
-03: 	depends on BR2_PACKAGE_HOST_RUSTC_ARCH_SUPPORTS
+03: 	depends on BR2_PACKAGE_HOST_RUSTC_TARGET_ARCH_SUPPORTS
 04: 	select BR2_PACKAGE_HOST_CARGO
 05: 	help
 06: 	  This is a comment that explains what foo is.