config doc: fix reference to config.worktree info

config doc: fix reference to config.worktree info

356aea6 ("doc: move extensions.worktreeConfig to the right place",
2018-11-14) moved the explanation of extension.worktreeConfig from
config.txt to technical/repository-version.txt. However, the former
still contains a reference to the removed paragraph. We could fix it
referencing the gitrepository-layout man page, which contains the moved
explanation. But the git-worktree man page has additional information
and recommendations for the worktree config file, so let's reference it
instead.

Signed-off-by: Matheus Tavares <matheus.bernardino@usp.br>
Signed-off-by: Junio C Hamano <gitster@pobox.com>