ruby docs: fix text and example (#28006)

1. 'wrapper' has been renamed to 'wrappedRuby', so use this instead.
2. mkDerivation isn't called with a 'src' attribute, so skip the 'unpackPhase' to avoid an error.
3. Simplify the build command. 'mkdir' and 'patchShebangs' don't need to be called explicitly.