Fix grammar generators to produce renderable include files in rst format

The missing `::` in the .rst files caused grammar section in docs to
render empty.

The `::` was accidentally removed in an unrelated commit
58bd26b6cf which was supposed to update
only copyright headers.

Fixes: #3120

doc/misc/rst-grammars.pl

@@ -39,6 +39,8 @@ print <<END;
 .. See the COPYRIGHT file distributed with this work for additional
 .. information regarding copyright ownership.
 
+::
+
 END
 
 # skip preamble

doc/misc/rst-zoneopt.pl

@@ -38,6 +38,8 @@ print <<END;
 .. See the COPYRIGHT file distributed with this work for additional
 .. information regarding copyright ownership.
 
+::
+
 END
 
 while (<FH>) {