[Docs] Remove non-existent --all flag from dstack stop

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: peterschmidt85

skills/dstack/SKILL.md

@@ -352,9 +352,6 @@ dstack stop my-run-name -y
 
 # Abort (force stop)
 dstack stop my-run-name --abort
-
-# Stop all runs
-dstack stop --all -y
 ```
 
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