spec: Drop "one or more" from index description

Fixing overly-strict wording from 0cc871e (spec: consistent wording,
#641).  In image-index.md, we explicitly allow 'manifests' to be
empty:

  While this property MUST be present, the size of the array MAY be
  zero.

Which allows you to do things like distribute an image-layout which
only provides CAS blobs.

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

Author: wking

spec.md

@@ -52,7 +52,7 @@ The [OCI Image Media Types](media-types.md) document is a starting point to unde
 The high-level components of the spec include:
 
 * [Image Manifest](manifest.md) - a document describing the components that make up a container image
-* [Image Index](image-index.md) - an annotated index of one or more image manifests
+* [Image Index](image-index.md) - an annotated index of image manifests
 * [Image Layout](image-layout.md) - a filesystem layout representing the contents of an image
 * [Filesystem Layer](layer.md) - a changeset that describes a container's filesystem
 * [Image Configuration](config.md) - a document determining layer ordering and configuration of the image suitable for translation into a [runtime bundle][runtime-spec]