Update secret name in README.md

[felipecruz91]

When following the instructions in the README.md, the deploy operation failed due to a missing secret.

$ faas-cli deploy
Deploying: submit-sample.
WARNING! You are not using an encrypted connection to the gateway, consider using HTTPS.

Unexpected status: 400, message: unable to find secret: influx-pass
unable to start task: submit-sample, error: OCI runtime create failed: container_linux.go:344: starting container process caused "process_linux.go:424: container init caused \"rootfs_linux.go:58: mounting \\\"/var/lib/faasd-provider/secrets/influx-pass\\\" to rootfs \\\"/run/containerd/io.containerd.runtime.v2.task/openfaas-fn/submit-sample/rootfs\\\" at \\\"/var/openfaas/secrets/influx-pass\\\" caused \\\"stat /var/lib/faasd-provider/secrets/influx-pass: no such file or directory\\\"\"": unknown

This is due to a different secret name being defined in the stack.yml file: https://github.com/alexellis/growlab/blob/master/data-logger/stack.yml#L20

[alexellis]

Thanks