descriptor: digest example link should be SHA-512

Also fixes a link to the algorithms.

Signed-off-by: Stephen J Day <stephen.day@docker.com>

descriptor.md

@@ -81,11 +81,11 @@ Some example digest strings include the following:
 digest                                                                    | algorithm           | Supported |
 --------------------------------------------------------------------------|---------------------|-----------|
 `sha256:6c3c624b58dbbcd3c0dd82b4c53f04194d1247c6eebdaab7c610cf7d66709b3b` | [SHA-256](#sha-256) | Yes       |
-`sha512:401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b372742...`   | [SHA-256](#sha-512) | Yes       |
+`sha512:401b09eab3c013d4ca54922bb802bec8fd5318192b0a75f201d8b372742...`   | [SHA-512](#sha-512) | Yes       |
 `multihash+base58:QmRZxt2b1FVZPNqd8hsiykDL3TdBDeTSPX9Kv46HmX4Gx8`         | Multihash           | No        |
 `sha256+b64u:LCa0a2j_xo_5m0U8HTBBNBNCLXBkg7-g-YpeiGJm564`                 | SHA-256 with urlsafe base64 | No|
 
-Please see [Registered Algorithms](#registered-identifiers) for a list of supported algorithms.
+Please see [Registered Algorithms](#registered-algorithms) for a list of supported algorithms.
 
 Implementations SHOULD allow digests that are unsupported to pass validation if they comply with the above grammar.
 While `sha256` will only use hex encoded digests, support for separators in _algorithm_ and alpha numeric in _encoded_ is included to allow for future extension of digest support.