readme: Fix relative URLs in README

Same story as in e.g.

    kirr/wendelin.core@b0b2c52e

( in short: GitLab now prepends namespace/repo/blob/ref/ prefix by
  itself )

README.rst

@@ -57,6 +57,5 @@ instance into/from git-backup managed repository. See `contrib/gitlab-backup`__
 for details.
 
 
-.. links are made so they show ok on project main page
-__ /kirr/git-backup/blob/master/git-backup
-__ /kirr/git-backup/blob/master/contrib/gitlab-backup
+__ git-backup
+__ contrib/gitlab-backup