image-layout: fix inconsistent description about external blob store

Following sentences were conflicting:

 * A blob, referenced with digest `<alg>:<hex>` (per [descriptor](descriptor.md#digests-and-verification)), MUST have its content stored in a file under `blobs/<alg>/<hex>`.
 * The blobs directory MAY be missing referenced blobs, in which case the missing blobs SHOULD be fulfilled by an external blob store.

Signed-off-by: Akihiro Suda <[email protected]>

Author: AkihiroSuda

image-layout.md

@@ -53,7 +53,7 @@ afff3924849e458c5ef237db5f89539274d5e609db5db935ed3959c90f1f2d51 ./blobs/sha256/
 ## Blobs
 
 * Object names in the `blobs` subdirectories are composed of a directory for each hash algorithm, the children of which will contain the actual content.
-* A blob, referenced with digest `<alg>:<hex>` (per [descriptor](descriptor.md#digests-and-verification)), MUST have its content stored in a file under `blobs/<alg>/<hex>`.
+* A blob, referenced with digest `<alg>:<hex>` (per [descriptor](descriptor.md#digests-and-verification)), SHOULD have its content stored in a file under `blobs/<alg>/<hex>`.
 * 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](#indexjson-file).