Documentation/config/pack.txt: fix broken AsciiDoc mark-up

Documentation/config/pack.txt: fix broken AsciiDoc mark-up

In af626ac0e0 (pack-bitmap: enable reuse from all bitmapped packs,
2023-12-14), the documentation for `pack.allowPackReuse` was amended to
include its effect when set to "multi".

This split the documentation into two paragraphs, but did not de-dent
the second paragraph on the right-hand side of a line-continuation
marker. This causes the rendered documentation to appear oddly, where
the second paragraph is treated as a <pre> block when rendered as HTML.

Fix this by correctly removing the indentation on the second paragraph.

Signed-off-by: Taylor Blau <me@ttaylorr.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>