README: Force use of named curves in example

MacOS uses LibreSSL for the OpenSSL command, and LibreSSL defaults to
using explicit curve parameters rather than named curves when encoding
private keys with `openssl req ...`. But the Golang x509 library does
not support explicit curve parameters, causing `tesla-http-proxy` to
fail with "x509: invalid ECDSA parameters".

This commit fixes the problem by adding an option to the openssl command
in the README that forces LibreSSL to use named curves.

README.md

@@ -185,7 +185,9 @@ purposes, you can create a self-signed localhost server certificate using
 OpenSSL:
 
 ```
-openssl req -x509 -nodes -newkey ec -pkeyopt ec_paramgen_curve:secp521r1 \
+openssl req -x509 -nodes -newkey ec \
+    -pkeyopt ec_paramgen_curve:secp521r1 \
+    -pkeyopt ec_param_enc:named_curve  \
     -subj '/CN=localhost' \
     -keyout key.pem -out cert.pem -sha256 -days 3650 \
     -addext "extendedKeyUsage = serverAuth" \