Remove doc reference to non-existing GCM example

Reviewed-by: Matt Caswell <matt@openssl.org>
Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2866)
(cherry picked from commit f6ff4e32d48f8929fa33781bff5e6e42c713564f)