mdadm/md.4: set page-length as 1000 to avoid warnings from grotty

mdadm/md.4: set page-length as 1000 to avoid warnings from grotty

"nroff" ("man") or ("tbl") needs a long page to avoid warnings
from "grotty", set 'pl' as 1000 to fix the following issue.

This command invokes by Makefile rule.
linux-tjrh:~/mdadm # man -l md.4 > test
grotty:<standard input> (<standard input>):5967:
character above first line discarded
... ...

Signed-off-by: Zhilong Liu <zlliu@suse.com>
Signed-off-by: Jes Sorensen <jsorensen@fb.com>