Copilot Chat: Update ReadMe to avoid confusion in starting the webapp (#1953)

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Address issue reported by
#1951.

### Description
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Add instructions to open a terminal and navigate to the webapp folder to
install and start the CC web app.

### Contribution Checklist
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- [ ] The code builds clean without any errors or warnings
- [ ] The PR follows SK Contribution Guidelines
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verified with `dotnet format`
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Co-authored-by: Aman Sachan <51973971+amsacha@users.noreply.github.com>

Author: TaoChenOSU

samples/apps/copilot-chat-app/README.md

@@ -107,7 +107,7 @@ First, let’s set up and verify the back-end API server is running.
       REACT_APP_SK_API_KEY={Your API Key, should be the same as Authorization:ApiKey from appsettings.json}
       ```
 
-   3. To build and run the front-end application
+   3. To build and run the front-end application, open a terminal and navigate to `samples/apps/copilot-chat-app/webapp` if not already, then run:
       ```bash
       yarn install
       yarn start