change ca path env. var in KMS guide (#9125)

This commit fixes the env. variable in the
KMS guide used to specify the CA certificates
for the KES server.

Before the env. variable `MINIO_KMS_KES_CAPATH` has
been used - which works in non-containerized environments
due to how MinIO merges the config file and environment
variables. In containerized environments (e.g. docker)
this does not work and trying to specify `MINIO_KMS_KES_CAPATH`
instead of `MINIO_KMS_KES_CA_PATH` eventually leads to MinIO not
trusting the certificate presented by the kes server.

See: cfd12914e1/cmd/crypto/config.go (L186)

docs/kms/README.md

@@ -401,7 +401,7 @@ export MINIO_KMS_KES_ENDPOINT=https://localhost:7373
 export MINIO_KMS_KES_KEY_FILE=minio.key
 export MINIO_KMS_KES_CERT_FILE=minio.cert
 export MINIO_KMS_KES_KEY_NAME=minio-key-1
-export MINIO_KMS_KES_CAPATH=kes-tls.crt
+export MINIO_KMS_KES_CA_PATH=kes-tls.crt
 ```
 > The `MINIO_KMS_KES_CAPATH` is only required since we use self-signed certificates.