libkmod: Improve index comment

libkmod: Improve index comment

In version 1 of kmod, definitions of INDEX_MAGIC_OLD and INDEX_MAGIC
were in two adjacent lines, which made the sentence starting with
"Second one" much easier to read.

Since the "second one" is actually INDEX_MAGIC and even kmod version 1
did use it, let's not talk about "newer versioned binary files" anymore
but clearly state that 0xB007FA57 is deprecated.

Also add a missing full stop.

Signed-off-by: Tobias Stoeckmann <tobias@stoeckmann.org>
Signed-off-by: Max Kunzelmann <maxdev@posteo.de>
Link: https://github.com/kmod-project/kmod/pull/241
Signed-off-by: Lucas De Marchi <lucas.de.marchi@gmail.com>