docs: update sendmail docs to use more secure SMTP server for Gmail

docs: update sendmail docs to use more secure SMTP server for Gmail

Earlier recommendation by IETF with RFC 2595 was to deprecate
implicit TLS in preference for upgrade an initially unencrypted
connection with STARTTLS command.  These days, however, IETF
recommends that connections be made using "Implicit TLS", in
preference to STARTTLS and the like, completely reversing their
earlier position, in RFC8314.

Update the GMail example to use the implicit TLS to match the
current recommendation at port 465.

Signed-off-by: Aditya Garg <gargaditya08@live.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>