src/bin/sortman: Fix order of intro(*) pages for subsections

src/bin/sortman: Fix order of intro(*) pages for subsections

Here's what changes with this patch:

	$ diff -u \
		<(find man -type f | ../master/src/bin/sortman) \
		<(find man -type f | ./src/bin/sortman);
	--- /dev/fd/63	2025-07-22 11:47:27.982307499 +0200
	+++ /dev/fd/62	2025-07-22 11:47:27.982307499 +0200
	@@ -805,7 +805,6 @@
	 man/man2const/VT_SETMODE.2const
	 man/man2const/VT_WAITACTIVE.2const
	 man/man2type/open_how.2type
	-man/man3attr/intro.3attr
	 man/man3/intro.3
	 man/man3/a64l.3
	 man/man3/abort.3
	@@ -2517,6 +2516,7 @@
	 man/man3/yn.3
	 man/man3/ynf.3
	 man/man3/ynl.3
	+man/man3attr/intro.3attr
	 man/man3attr/gnu::aligned.3attr
	 man/man3attr/gnu::format.3attr
	 man/man3const/EOF.3const

Signed-off-by: Alejandro Colomar <alx@kernel.org>