doc txt & -h consistency: balance unbalanced "[" and "]"

doc txt & -h consistency: balance unbalanced "[" and "]"

Fix a "-h" output syntax issue introduced when "--diagnose" was added
in aac0e8ffeee (builtin/bugreport.c: create '--diagnose' option,
2022-08-12): We need to close the "[" we opened. The
corresponding *.txt change did not have the same issue.

The "help -h" output then had one "]" too many, which is an issue
introduced in b40845293b5 (help: correct the usage string in -h and
documentation, 2021-09-10).

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>