fix variable declaration for date parse/format docs

Author: ap8351

docs/guide.html

@@ -901,8 +901,8 @@ <h3 id="masked-date" class="section-h"><a href="#masked-date">Date Mask</a></h3>
 
   // define date -> str convertion
   format: date => {
-    const day = date.getDate();
-    const month = date.getMonth() + 1;
+    let day = date.getDate();
+    let month = date.getMonth() + 1;
     const year = date.getFullYear();
 
     if (day < 10) day = "0" + day;