image-layout: Replace "slash separated layout" with "directory structure"

Adjusting wording we've been carrying since 9bb56d8 (image-layout:
add an initial image layout spec, 2016-05-26, opencontainers#605).

On Windows, an unpacked layout will use backslashes.  Layouts really
don't care what the directory separator is, and each access method
(filesytem access, tar unpacker, etc.) already has an unambiguous
separator character.

Signed-off-by: W. Trevor King <[email protected]>

Author: wking

image-layout.md

@@ -1,6 +1,6 @@
 ## OCI Image Layout Specification
 
-* The OCI Image Layout is a slash separated layout of OCI content-addressable blobs and [location-addressable](https://en.wikipedia.org/wiki/Content-addressable_storage#Content-addressed_vs._location-addressed) references (refs).
+* The OCI Image Layout is directory structure for OCI content-addressable blobs and [location-addressable](https://en.wikipedia.org/wiki/Content-addressable_storage#Content-addressed_vs._location-addressed) references (refs).
 * This layout MAY be used in a variety of different transport mechanisms: archive formats (e.g. tar, zip), shared filesystem environments (e.g. nfs), or networked file fetching (e.g. http, ftp, rsync).
 
 Given an image layout and a ref, a tool can create an [OCI Runtime Specification bundle](https://github.com/opencontainers/runtime-spec/blob/v1.0.0/bundle.md) by: