examples/rust: Fix building of the hello world demo

When I renamed it from minimal to hello-world, it stopped being built
due to the make target name being the same as the directory name
(hello-world).

Rename the make target to rust-hello-world which also matches the naming
of the rest of the targets.

Fixes: 656c036 ("examples/rust: Add a minimal hello world rust example")
Signed-off-by: Andrew Clayton <a.clayton@nginx.com>

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