Change the external link

[arata-honda]

I commit on #2235, but build is error.

- ./_site/scala3/book/why-scala-3.html
  *  External link https://dotty.epfl.ch/docs/Dropped%20Features/index.html failed: 404 No error
  *  External link https://dotty.epfl.ch/docs/Other%20Changed%20Features/index.html failed: 404 No error
htmlproofer 3.15.3 | Error:  HTML-Proofer found 3 failures!
  *  External link https://dotty.epfl.ch/docs/Other%20New%20Features/index.html failed: 404 No error

I try to visit this links but link is changed

• https://dotty.epfl.ch/docs/Dropped%20Features/index.html
  -> https://dotty.epfl.ch/docs/dropped%20features/index.html
• https://dotty.epfl.ch/docs/Other%20Changed%20Features/index.html
  -> https://dotty.epfl.ch/docs/other%20changed%20features/index.html
• https://dotty.epfl.ch/docs/Other%20New%20Features/index.html
  -> https://dotty.epfl.ch/docs/other%20new%20features/index.html

maybe these urls changed from uppercase to lowercase.