net: core: Correct function name netevent_unregister_notifier() in the kerneldoc

Fix the following W=1 kernel build warning(s):

 net/core/netevent.c:45: warning: expecting prototype for netevent_unregister_notifier(). Prototype was for unregister_netevent_notifier() instead

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Signed-off-by: David S. Miller <davem@davemloft.net>