fixed typo on UInt64

Author: Steven Liu

src/model/UInt64.ts

@@ -38,7 +38,7 @@ export class UInt64 {
      */
     public static fromUint(value: number): UInt64 {
         if (value < 0) {
-            throw new Error('Unit value cannot be negative');
+            throw new Error('Unsigned integer cannot be negative');
         }
         return new UInt64(uint64.fromUint(value));
     }

test/model/UInt64.spec.ts

@@ -67,7 +67,7 @@ describe('Uint64', () => {
     it('should fromUnit throw exception with negative unit value', () => {
         expect(() => {
             UInt64.fromUint(-1);
-        }).to.throw(Error, 'Unit value cannot be negative');
+        }).to.throw(Error, 'Unsigned integer cannot be negative');
     });
 
     describe('equal', () => {