docs: update help file

Author: franco-ruggeri

doc/pdf-preview.txt

@@ -1,4 +1,4 @@
-*pdf-preview.txt*           For NVIM v0.8.0          Last change: 2025 June 29
+*pdf-preview.txt*           For NVIM v0.8.0          Last change: 2025 July 07
 
 ==============================================================================
 Table of Contents                              *pdf-preview-table-of-contents*
@@ -141,7 +141,7 @@ request once agreed.
 ==============================================================================
 9. Links                                                   *pdf-preview-links*
 
-1. *demo*: ./assets/demo.gif
+1. *demo*: https://github.com/user-attachments/assets/76bd5ff4-aade-4484-85fc-380f40c8cdf0
 
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