doc: column: fix blank lines around block delimiters

doc: column: fix blank lines around block delimiters

227c4f33a03 (doc: add a blank line around block delimiters,
2025-03-09) added blank lines around block delimiters as a
defensive measure.  For each block you had to mind the con-
text (like the commit says):

• Top-level: just add blank lines
• Block: use list continuation (+)

But list continuation was used here at the top level, which
results in literal `+` in the output formats.

Acked-by: Jean-Noël Avila <jn.avila@free.fr>
Signed-off-by: Kristoffer Haugsbakk <code@khaugsbakk.name>
Signed-off-by: Junio C Hamano <gitster@pobox.com>