Update nightly version in CONTRIBUTING.md to 2023-02-17

Summary:
D13145 bumped the nightly version, update the instructions in CONTRIBUTING.md accordingly.

Note: To uninstall the old nightly version, run rustup toolchain uninstall nightly-2022-06-29

Test Plan: Paste the new install instructions for the "abc-nightly" toolchain into a command line (should be done for future diffs anyway).

Reviewers: Fabien, #bitcoin_abc

Reviewed By: Fabien, #bitcoin_abc

Differential Revision: https://reviews.bitcoinabc.org/D13146