fix grammar in 'not enough unspent tx outputs' error in tx builder

lib/TransactionBuilder.js

@@ -283,7 +283,7 @@ TransactionBuilder.prototype._selectUnspent = function(neededAmountSat) {
   } while (!fulfill && minConfirmationSteps.length);
 
   if (!fulfill)
-    throw new Error('no enough unspent to fulfill totalNeededAmount [SAT]:' +
+    throw new Error('not enough unspent tx outputs to fulfill totalNeededAmount [SAT]:' +
                     neededAmountSat);
 
   this.selectedUtxos = sel;