Documentation: Fix intiramfs script name

Documentation was not changed when renaming the script in commit
80e715a0 ("initramfs: rename gen_initramfs_list.sh to
gen_initramfs.sh"). Fixing this.

Basically does:

 $ sed -i -e s/gen_initramfs_list.sh/gen_initramfs.sh/g $(git grep -l gen_initramfs_list.sh)

Fixes: 80e715a0

 ("initramfs: rename gen_initramfs_list.sh to gen_initramfs.sh")
Signed-off-by: Robert Richter <rrichter@amd.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>