Add an entry to the CHANGES for the d2i_X509_PUBKEY fix

The commit 5dc40a83c74be579575a512b30d9c1e0364e6a7b forgot
to add a short description to the CHANGES file.

Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/8144)

(cherry picked from commit b2aea0e3d9a15e30ebce8b6da213df4a3f346155)