doc/go1.14: fix broken links
    
    Fix broken links with missing /pkg and fix os/signal import path.
    
    Change-Id: I4bf74c9f6e9b0e5fa96ca4d35c4ce404e07583b2
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217697
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>