MAINTAINERS: adjust file entry in CPU HOTPLUG

Commit c7f005f7 ("rust: cpu: Add CpuId::current() to retrieve current
CPU ID") adds the file rust/helpers/cpu.c, and intends to add a file entry
for that file in the MAINTAINERS section CPU HOTPLUG. However, the added
file entry is rust/helper/cpu.c; note the subtle difference between the two
file names. Hence, ./scripts/get_maintainer.pl --self-test=patterns
complains about a broken reference.

Adjust the file entry to the intended file.

Fixes: c7f005f7 ("rust: cpu: Add CpuId::current() to retrieve current CPU ID")
Signed-off-by: Lukas Bulwahn <lukas.bulwahn@redhat.com>
Reviewed-by: Boqun Feng <boqun.feng@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>