Update 2.1.md

[Fix] Update to solution for 2.1-4

The previous solution has a couple mistakes involved (utilizing the loop variable beyond its block, incorrect output). I wrote up the issue report via this link: walkccc#437

docs/Chap02/2.1.md

@@ -75,11 +75,15 @@ LINEAR-SEARCH(A, v)
 
 ```cpp
 ADD-BINARY(A, B)
+    r = 0
     C = new integer[A.length + 1]
-    carry = 0
-    for i = 1 to A.length
-        C[i] = (A[i] + B[i] + carry) % 2  // remainder
-        carry = (A[i] + B[i] + carry) / 2 // quotient
-    C[i + 1] = carry
+    for i = A.length to 1 do
+        s = A[i] + B[i] + r
+        C[i + 1] = s % 2
+        if s > 1
+            r = 1
+        else
+            r = 0
+    C[1] = r
     return C
 ```