Update Strings module: Replace russian text with Ukrainian

[spragaa]

trafficstars

This pull request updates the examples in the Strings module of the Rust book to use Ukrainian language instead of russian. As a Ukrainian citizen, I believe it is important to promote and popularize the Ukrainian language, especially given the current geopolitical context.

Changes:

• Replaced russian text examples with equivalent Ukrainian text.
• Updated explanations and comments to reflect the changes in examples.

Reasoning:

• Given the ongoing war and the role of russia as an aggressor state, it is important to avoid the normalization of their language in educational materials.

Thank you for considering this contribution. I look forward to your feedback and hope to see this change integrated into the Rust book.