use home dir for terraform plugin

[qayshp]

~/.terraform.d/plugins rather than arbitrary install directory

What does this pull request do? Update the docs page to help better with the terraform plugin installation.

How should this be tested? Try the instructions now :)

Checklist

• [ ] ~Unit tests~
• [ ] ~Smoke tests (if applicable)~
• [ ] ~System tests (if applicable)~
• [X] Documentation (if applicable)

closes # No related issue.

[qayshp]

Not sure if this really matters anymore, but, ¯\_(ツ)_/¯