update in visualizing trace option (#2855)

`--visualize` option does not work, it needs to include
`--enable-unstable` as well.
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docs/src/tutorial-first-steps.md

@@ -66,7 +66,7 @@ A trace is a record of exactly how execution proceeded, including concrete choic
 To get a trace for a failing check in Kani, run:
 
 ```
-cargo kani --visualize
+cargo kani --visualize --enable-unstable
 ```
 
 This command runs Kani and generates an HTML report that includes a trace.