doc: Node exporter download instructions clarification

[mkbhanda]

on https://prometheus.io/docs/guides/node-exporter/

1. Add text to alert the user to down the bundle appropriate to their operating system and hardware architecture.

2. Add a few more wild cards int eh tar bundle names to cover architecture.

wget https://github.com/prometheus/node_exporter/releases/download/v*/node_exporter-*.*-amd64.tar.gz
tar xvfz node_exporter-.-amd64.tar.gz

[statickidz]

I can take this @hdost

Can you please assign it to me?

Thanks!