Rename "private boxes" to "protected boxes"

[jsejcksn]

This is an idea to improve the messaging/documentation.

The description of "private" boxes might be misleading to some because they can be read without the API key. "Protected" might be a better description because only mutation actions (create/update/delete) require an API key.

[vasanthv]

Yeah, you are right protected is much better word for this. We can change that.

[jsejcksn]

@vasanthv I still see "private" in the repo description:

[vasanthv]

Fixed it. Thanks for pointing it out.