repo: remove blank line from Documentation/git-repo.adoc

repo: remove blank line from Documentation/git-repo.adoc

There was an extra blank line in git-repo-structure documentation, which
led to an unwawnted '+' character after generating an HTML or PDF from
that page. This can be seen, for example, in Git 2.52.0 online docs [1].

Remove that extra line.

[1] https://git-scm.com/docs/git-repo/2.52.0

Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>