fix: block CID links (#356)

License: MIT
Signed-off-by: Alan Shaw <alan@tableflip.io>

Author: alanshaw

SPEC/BLOCK.md

@@ -159,3 +159,4 @@ A great source of [examples][] can be found in the tests for this API.
 [block]:https://github.com/ipfs/js-ipfs-block
 [multihash]:https://github.com/multiformats/multihash
 [examples]: https://github.com/ipfs/interface-ipfs-core/blob/master/js/src/block
+[cid]: https://www.npmjs.com/package/cids