Documentation/filesystems: ramfs-rootfs-initramfs: use :Author:

Use the :Author: markup instead of making it a chapter heading.
This cleans up the table of contents for this file.

Fixes: 7f46a240

 ("[PATCH] ramfs, rootfs, and initramfs docs")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Rob Landley <rob@landley.net>
Reviewed-by: Christian Brauner <brauner@kernel.org>
Link: https://lore.kernel.org/r/20230508055928.3548-1-rdunlap@infradead.org
Signed-off-by: Jonathan Corbet <corbet@lwn.net>