docs: fix link to plugin development docs (#6198)

Fixes the link to the plugin development documentation.

Author: semohr

README.rst

@@ -85,7 +85,7 @@ simple if you know a little Python.
 
 .. _transcode audio: https://beets.readthedocs.org/page/plugins/convert.html
 
-.. _writing your own plugin: https://beets.readthedocs.org/page/dev/plugins.html
+.. _writing your own plugin: https://beets.readthedocs.org/page/dev/plugins/index.html
 
 Install
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README_kr.rst

@@ -79,7 +79,7 @@ Beets는 라이브러리로 디자인 되었기 때문에, 당신이 음악들
 
 .. _transcode audio: https://beets.readthedocs.org/page/plugins/convert.html
 
-.. _writing your own plugin: https://beets.readthedocs.org/page/dev/plugins.html
+.. _writing your own plugin: https://beets.readthedocs.org/page/dev/plugins/index.html
 
 설치
 -------