fix regular expression of algorithim

change regular expression of algorithim in defs-image.json to:
	"pattern": "^[a-z0-9_+.-]+"
and make descriptor.md consistent with it.

Signed-off-by: Deng Guangxing <dengguangxing@huawei.com>

descriptor.md

@@ -60,7 +60,7 @@ The digest string matches the following grammar:
 
 ```
 digest      := algorithm ":" hex
-algorithm   := /[A-Fa-f0-9_+.-]+/
+algorithm   := /[a-z0-9_+.-]+/
 hex         := /[A-Fa-f0-9]+/
 ```
 

schema/defs-image.json

@@ -9,7 +9,7 @@
     "digest": {
       "description": "the cryptographic checksum digest of the object, in the pattern '<hash>:<hexadecimal digest>'",
       "type": "string",
-      "pattern": "^[a-z0-9]+:[a-fA-F0-9]+$"
+      "pattern": "^[a-z0-9_+.-]+:[a-fA-F0-9]+$"
     },
     "manifestDescriptor": {
       "id": "https://opencontainers.org/schema/image/manifestDescriptor",