Replace occurrences of "private key" with "mnemonic seed phrase" in seed_qr README.md

[jambolo]

Replace occurrences of "private key" with "mnemonic seed phrase" in seed_qr README.md.

Description

The seed_qr README.md occasionally uses the term "private key" when "mnemonic seed phrase" is the correct term.

This pull request is categorized as a:

• [ ] New feature
• [ ] Bug fix
• [ ] Code refactor
• [x] Documentation
• [ ] Other

Checklist

• [ ] I’ve run pytest and made sure all unit tests pass before sumbitting the PR

If you modified or added functionality/workflow, did you add new unit tests?

• [ ] No, I’m a fool
• [ ] Yes
• [x] N/A

I have tested this PR on the following platforms/os:

• [ ] Raspberry Pi OS Manual Build
• [ ] SeedSigner OS on a Pi0/Pi0W board
• [ ] Other

Note: Keep your changes limited in scope; if you uncover other issues or improvements along the way, ideally submit those as a separate PR. The more complicated the PR the harder to review, test, and merge.

[newtonick]

I believe "private key" in this context was used loosely to represent a secret that is stored and entered into a Bitcoin signing device. This said I'm good with this change. I do think it's technically more accurate. @kdmukai this was mostly your original work and better at writing/expressing documentation than I am. Your thoughts?