translate the "mptcp-sysctl.rst" into Simplified Chinese.
Update the translation through commit fa3ee9dd80
("mptcp: sysctl: add available_path_managers")
Signed-off-by: Sun yuxi <sun.yuxi@zte.com.cn>
Reviewed-by: xu xin <xu.xin16@zte.com.cn>
Signed-off-by: Alex Shi <alexs@kernel.org>
