Add header option for the tables

Approved by:	carlavilla (implicitly)

website/content/en/releases/11.4R/schedule.adoc

@@ -22,7 +22,7 @@ link:{handbook}#mfc-glossary[MFC] requests are no longer being accepted.
 == Schedule
 
 [.tblbasic]
-[cols=",,,",]
+[cols=",,,",options="header",]
 |===
 |Action |Expected |Actual |Description
 |Initial release schedule announcement |- |3 January 2020 |Release Engineers send announcement email to developers with a rough schedule.

website/content/en/releases/12.2R/schedule.adoc

@@ -23,7 +23,7 @@ General discussions about the pending release and known issues should be sent to
 == Schedule
 
 [.tblbasic]
-[cols=",,,",]
+[cols=",,,",options="header",]
 |===
 |Action |Expected |Actual |Description
 |Initial release schedule announcement |- |3 January 2020 |Release Engineers send announcement email to developers with a rough schedule.

website/content/en/releases/13.0R/schedule.adoc

@@ -29,7 +29,7 @@ During the {localBranchHead} freeze, the Release Engineering Team may create ALP
 Announcements regarding the availability of the ALPHA snapshots will be sent to the mailto:freebsd-current@FreeBSD.org[freebsd-current] and mailto:freebsd-snapshots@FreeBSD.org[freebsd-snapshots] mailing lists.
 
 [.tblbasic]
-[cols=",,,",]
+[cols=",,,",options="header",]
 |===
 |Action |Expected |Actual |Description
 |Initial release schedule announcement |- |21 July 2020 |Release Engineers send announcement email to developers with a rough schedule.