fix a type error caused by (my) invalid API use

lib/commands/block.js

@@ -88,7 +88,7 @@ Block.validateAncestorProof = function (Env, proof, _cb) {
             }
             // else fall through to next step
         }).nThen(function () {
-            Block.check(Env, pub, function (err) {
+            BlockStore.check(Env, pub, function (err) {
                 if (err) { return void cb('E_MISSING_ANCESTOR'); }
                 cb(void 0, pub);
             });