fix copy_extensions config name

ending `s` was missing from the copy_extensions

as mentioned, config file should have `copy_extensions = copy` in CA_default section for openssl ca to copy extensions from req.

Author: sku0x20

sign-server-and-client-certificates.rst

@@ -69,7 +69,7 @@ and reference it in ``server_cert`` section.
     DNS.3          = m.example.com
 
 In order to keep this extension when the intermediate
-CA signs this CSR, ``copy_extension = copy`` must be
+CA signs this CSR, ``copy_extensions = copy`` must be
 present in section ``[ CA_default ]`` in
 ``intermediate/openssl.cnf``.
 
@@ -218,4 +218,4 @@ certificates using the ``-certfile`` option.
 
     # openssl pkcs12 -export -out intermediate/certs/client.full.pfx \
         -inkey intermediate/private/client.key.pem -in intermediate/certs/client.cert.pem \
-        -certfile intermediate/certs/intermediate.cert.pem -certfile certs/ca.cert.pem
+        -certfile intermediate/certs/intermediate.cert.pem -certfile certs/ca.cert.pem