[GRASS-git] [OSGeo/grass-website] a75967: fix broken link

[Markus Neteler]

  Branch: refs/heads/master
  Home:   https://github.com/OSGeo/grass-website
  Commit: a75967f89d5cf108d176c4fd57a67d009a7f706d
      https://github.com/OSGeo/grass-website/commit/a75967f89d5cf108d176c4fd57a67d009a7f706d
  Author: Markus Neteler <neteler at gmail.com>
  Date:   2020-07-28 (Tue, 28 Jul 2020)

  Changed paths:
    M content/learn/books.md

  Log Message:
  -----------
  fix broken link