std-docs: use `open` for macOS.

[jiacai2050]

$ zig std 
http://127.0.0.1:59993/
error: FileNotFound

0.12.0 will output an error on macOS, it should be open.

• https://superuser.com/questions/911735/how-do-i-use-xdg-open-from-xdg-utils-on-mac-osx

Also, this PR make /debug endpoint works, previously it will return 404...