doc/install: update install guide with 0.8.1 version
```sh
# Set the release version variable
-$ RELEASE_VERSION=v0.8.0
+$ RELEASE_VERSION=v0.8.1
# Linux
$ curl -OJL https://github.com/operator-framework/operator-sdk/releases/download/${RELEASE_VERSION}/operator-sdk-${RELEASE_VERSION}-x86_64-linux-gnu
# macOS
