Small updates to CLI docs

Thank you for this contribution to docs.nersc.gov!

Remove reference to deprecated Spin login node and update location of CLI download.

Submitters and reviewers please make sure

• internal links are relative and to the .md file
• avoid links named "here"
• alt-text for images is descriptive
• doesn't duplicate existing content
• spell check
• renders correctly
• does not include any identifiable information (real usernames, project names, etc)