image-layout: Link "refs" to the index.json section

To make it clear what they are now that we have no "Refs" section
(which was removed in 0556a6b, image-layout: ./refs/ -> index.json,
2017-01-26, #533).

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

Author: wking

image-layout.md

@@ -58,7 +58,7 @@ A blob, referenced with digest `<alg>:<hex>` (per [descriptor](descriptor.md#dig
 The character set of the entry name for `<hex>` and `<alg>` MUST match the respective grammar elements described in [descriptor](descriptor.md#digests-and-verification).
 For example `sha256:5b` will map to the layout `blobs/sha256/5b`.
 
-The blobs directory MAY contain blobs which are not referenced by any of the refs.
+The blobs directory MAY contain blobs which are not referenced by any of the [refs](#indexjson-file).
 
 The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.