MAINTAINERS: Correct path to time namespace source file

According to reviews, Time Namespace source was moved from
kernel/time_namespace.c to kernel/time/namespace.c between patchset
versions, while the path in MAINTERNERS file wasn't adjusted properly.

Correct it, so get_maintainer.pl produces a correct emails list again.

Fixes: 769071ac

 ("ns: Introduce Time Namespace")
Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20200127143748.268515-1-dima@arista.com