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>

Documentation/config/pack.txt

@@ -34,11 +34,10 @@ pack.allowPackReuse::
 	reachability bitmap is available, pack-objects will try to send
 	parts of all packs in the MIDX.
 +
-	If only a single pack bitmap is available, and
-	`pack.allowPackReuse` is set to "multi", reuse parts of just the
-	bitmapped packfile. This can reduce memory and CPU usage to
-	serve fetches, but might result in sending a slightly larger
-	pack. Defaults to true.
+If only a single pack bitmap is available, and `pack.allowPackReuse`
+is set to "multi", reuse parts of just the bitmapped packfile. This
+can reduce memory and CPU usage to serve fetches, but might result in
+sending a slightly larger pack. Defaults to true.
 
 pack.island::
 	An extended regular expression configuring a set of delta