update flarectl install instructions for 1.17+
Closes
# flarectl
-A CLI application for interacting with a Cloudflare account. Powered by [cloudflare-go].
+A CLI application for interacting with a Cloudflare account. Powered by [cloudflare-go](https://github.com/cloudflare/cloudflare-go).
## Installation
Install it when you install our command-line library:
```sh
-go get -u github.com/cloudflare/cloudflare-go/...
+go install github.com/cloudflare/cloudflare-go/cmd/flarectl@latest
```
# Usage
