Use EXAMPLES not EXAMPLE for section title

Use EXAMPLES not EXAMPLE for section title

And update find-doc-nits to complain if "=head1 EXAMPLE" is found.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matthias St. Pierre <Matthias.St.Pierre@ncp-e.com>
(Merged from https://github.com/openssl/openssl/pull/9602)