Misc fixes

[Stephen-at-Pleno]

• I have searched open and closed issues and pull requests for duplicates, using these search terms:
  • page_title_for, Chapter 20, twitter
• I have checked the latest main branch to see if this has already been fixed, in this file:
  -/src/ch17-01-futures-and-syntax.md
  -/src/ch20-00-advanced-features.md
  -/src/end-of-experiment.md

URLs and proposed fixes:
https://rust-book.cs.brown.edu/ch20-00-advanced-features.html

• Replace "one more project in Chapter 20" with "one more project in Chapter 21"

https://rust-book.cs.brown.edu/ch17-01-futures-and-syntax.html

• Replace two references to page_url_for with correct function name page_url

https://rust-book.cs.brown.edu/end-of-experiment.html

• Consider replacing with You can follow [@tonofcrates](https://bsky.app/profile/tonofcrates.bsky.social) on BlueSky with You can follow [@tonofcrates](https://twitter.com/tonofcrates) on Twitter - up to you on whether you want to keep the twitter / X link instead.