docs: update setting-up.md (#6492)

* Remove `npm install` from steps

docs/development/setting-up.md

@@ -25,10 +25,9 @@ Unsupported Node.js LTS that should still work:
 
 1. `git clone https://github.com/swagger-api/swagger-ui.git`
 2. `cd swagger-ui`
-3. `npm install`
-4. `npm run dev`
-5. Wait a bit
-6. Open http://localhost:3200/
+3. `npm run dev`
+4. Wait a bit
+5. Open http://localhost:3200/
 
 ### Using your own local api definition with local dev build