Update sign.md

Demo中的Seed在经过Repeat（count参数为2）后应当为naOC0ocQE3shWLAfffVLB1rhYPG7naOC0ocQE3shWLAfffVLB1rhYPG7，值为naOC0ocQE3shWLAfffVLB1rhYPG7naOC的实际是rand

Author: Lucky-Qu

docs/develop/api-v2/dev-prepare/interface-framework/sign.md

@@ -36,7 +36,8 @@ DEMO
 ```
 【输入】
 secret: naOC0ocQE3shWLAfffVLB1rhYPG7
-seed: naOC0ocQE3shWLAfffVLB1rhYPG7naOC
+seed: naOC0ocQE3shWLAfffVLB1rhYPG7naOC0ocQE3shWLAfffVLB1rhYPG7
+rand: naOC0ocQE3shWLAfffVLB1rhYPG7naOC
 【输出】
 publicKey: [215 195 98 254 120 174 248 31 242 50 135 180 147 98 139 93 176 42 60 79 227 11 33 94 77 25 96 155 93 118 103 58]
 privateKey: [110 97 79 67 48 111 99 81 69 51 115 104 87 76 65 102 102 102 86 76 66 49 114 104 89 80 71 55 110 97 79 67 215 195 98 254 120 174 248 31 242 50 135 180 147 98 139 93 176 42 60 79 227 11 33 94 77 25 96 155 93 118 103 58]