[Docs] minor update to install instructions needed

[eburgueno]

Docs need updating after #403:

$ shpc config set registry:~/shpc/registry
You cannot use 'set' for a list. Use add/remove instead.

$ shpc config add registry:~/shpc/registry
Warning: Check with shpc config edit - ordering of list can change.
Added registry to ~/shpc/registry

[vsoch]

Yes indeed! Would you care to contribute a PR at your convenience?

[eburgueno]

Can do, thought it seems wasteful to create a commit (and PR) to replace three characters. I'll try looking at the docs a little bit more for other enhancements.

[vsoch]

Haha ok (but I don’t think it’s that wasteful!) every little improvement is a step in the right direction, whether addition or subtraction.

[vsoch]

@eburgueno just checking in - are you still interested to contribute here?

[eburgueno]

@vsoch yes, sorry, it's been a busy time. I'll send something shortly.

[vsoch]

No worries! See the linked PR for my tiny update to address what you found - if there is more please open a PR, if not apologies for taking me so long to fix that.

[eburgueno]

That looks just like what I was about to suggest. Thank you for beating me to it!