fix broken documentation links

docs/usage/headers-and-footers.md

@@ -23,7 +23,7 @@ doc.Header.createImage([BUFFER_OF_YOUR_IMAGE]);
 doc.Footer.createImage([BUFFER_OF_YOUR_IMAGE]);
 ```
 
-Refer to `demo8.js` for more information
+Refer to [`demo8.ts`](https://github.com/dolanmiu/docx/blob/master/demo/demo8.ts) for more information.
 
 ## Multiple Headers and Footers
 
@@ -38,10 +38,10 @@ Also all the supported section properties are implemented according to: http://o
     // Add new section with another header and footer
     doc.addSection({
       headers: {
-        default: header 
+        default: header
       },
       footers: {
-        default: footer 
+        default: footer
       },
       pageNumberStart: 1,
       pageNumberFormatType: docx.PageNumberFormat.DECIMAL,