[gimp-web] Fix linking for one link that was wrong. This is a old link not sure if its still the norm to write

[Jehan <jehanp src gnome org>]

commit 4df1a00dfa7622a88f00437559ad10b24409e6bb
Author: nmat <nmat users noreply github com>
Date:   Sat Jul 17 10:44:43 2021 +0200

    Fix linking for one link that was wrong.
    This is a old link not sure if its still the norm to write plugins like that.

 content/docs/index.md | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/content/docs/index.md b/content/docs/index.md
index 5f97789b..62a63bb2 100644
--- a/content/docs/index.md
+++ b/content/docs/index.md
@@ -58,7 +58,7 @@ The user manuals for older releases can be found at the [GIMP Documentation page
 ## GIMP Documentation for Developers and Script Authors
 
 *   [wiki.gimp.org](https://wiki.gimp.org/)
-*   [Writing a plug-in](plug-in/plug-in.html)
+*   [Writing a plug-in](http://gimp-plug-ins.sourceforge.net/doc/Writing/html/plug-in.html)
 *   [GIMP Python module documentation](python/index.html)
 *   [GIMP Script-Fu documentation](scheme_plugin/)
 *   [GIMP Script-Fu changes in GIMP 2.4 (migration guide)](script-fu-update.html)