doc: fix t0450-txt-doc-vs-help to select only first synopsis block

doc: fix t0450-txt-doc-vs-help to select only first synopsis block

In case there are multiple synopsis blocks (declared with [synopsis]
or [verse] style) in the same file, the previous implementation was
incorrectly picking up text from all the blocks until the first empty
line. This commit modifies the sed command to stop processing upon
encountering the first empty line after the first block declaration,
thereby ensuring that only the intended block is captured.

Signed-off-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>