manifest(-list): Fix "object" -> "string" for mediaType

I typo'd these in b1f1d9c (manifest: List media types supported by
descriptor references, 2016-09-13, opencontainers#304).

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

Author: wking

manifest-list.md

@@ -26,7 +26,7 @@ This section defines the `application/vnd.oci.image.manifest.list.v1+json` [medi
 
   Each object in the manifest is a [descriptor](descriptor.md) with the following additional properties and restrictions:
 
-  - **`mediaType`** *object*
+  - **`mediaType`** *string*
 
     This [descriptor property](descriptor.md#properties) has additional restrictions for `manifests`.
     Implementations MUST support at least the following media types:

manifest.md

@@ -30,7 +30,7 @@ Unlike the [Manifest List](manifest-list.md), which contains information about a
     This REQUIRED property references a configuration object for a container, by digest.
     Beyond the [descriptor requirements](descriptor.md#properties), the value has the following additional restrictions:
 
-    - **`mediaType`** *object*
+    - **`mediaType`** *string*
 
         This [descriptor property](descriptor.md#properties) has additional restrictions for `config`.
         Implementations MUST support at least the following media types:
@@ -48,7 +48,7 @@ Unlike the [Manifest List](manifest-list.md), which contains information about a
 
     Beyond the [descriptor requirements](descriptor.md#properties), the value has the following additional restrictions:
 
-    - **`mediaType`** *object*
+    - **`mediaType`** *string*
 
         This [descriptor property](descriptor.md#properties) has additional restrictions for `layers[]`.
         Implementations MUST support at least the following media types: