doc: checkout: remove duplicate synopsis

doc: checkout: remove duplicate synopsis

It was added in [1]. I understand that the duplicate change was not
intentional and comes from an oversight.

Also, in explanation, there was only one section for two synopsis
entries.

Fix both problems by removing duplicate synopsis.

<paths> vs <pathspec> is resolved in next patch.

[1] Commit b59698ae ("checkout doc: clarify command line args for "checkout paths" mode" 2017-10-11)

Signed-off-by: Alexandr Miloslavskiy <alexandr.miloslavskiy@syntevo.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>