doc/go1.14: quote {{ and }} in text/template note
    
    Fixes #37070
    
    Change-Id: I543957df264367e56c71a25bfaea5cf7935d438f
    Reviewed-on: https://go-review.googlesource.com/c/go/+/217979
    Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>