fix error message

lib/ecdsa.js

@@ -60,7 +60,7 @@ ECDSA.prototype.randomK = function() {
 ECDSA.prototype.sig2pubkey = function() {
   var i = this.sig.i;
   if (!(i === 0 || i === 1 || i === 2 || i === 3))
-    throw new Error('signature: i must be equal to 0, 1, 2, or 3');
+    throw new Error('i must be equal to 0, 1, 2, or 3');
 
   var e = BN().fromBuffer(this.hashbuf);
   var r = this.sig.r;