Update docs/advanced-usage/custom-instructions.md

removed .md extent ion from link

Co-authored-by: ellipsis-dev[bot] <65095814+ellipsis-dev[bot]@users.noreply.github.com>

Author: Michaelzag

docs/advanced-usage/custom-instructions.md

@@ -84,7 +84,7 @@ While related, custom instructions and custom rules serve slightly different pur
   - Often focused on code standards and practices
   - Typically organized by categories
 
-For detailed information on creating effective structured rules in rule files, see the [Custom Rules](custom-rules.md) documentation.
+For detailed information on creating effective structured rules in rule files, see the [Custom Rules](custom-rules) documentation.
 
 ## Examples of Custom Instructions