[docs] Fix links in the trading example (MystenLabs#16372)

## Description 

Fixed links to github

## Test Plan 

How did you test the new or updated feature?

---
If your changes are not user-facing and do not break anything, you can
skip the following section. Otherwise, please briefly describe what has
changed under the Release Notes section.

### Type of Change (Check all that apply)

- [ ] protocol change
- [ ] user-visible impact
- [ ] breaking change for a client SDKs
- [ ] breaking change for FNs (FN binary must upgrade)
- [ ] breaking change for validators or node operators (must upgrade
binaries)
- [ ] breaking change for on-chain data layout
- [ ] necessitate either a data wipe or data migration

### Release notes

docs/content/guides/developer/app-examples/trading/frontend.mdx

@@ -194,7 +194,7 @@ Use the `useSuiClient()` hook from `dApp Kit` to retrieve the Sui client instanc
 
 :::info
 
-The full source code of the demo bear smart contract is available at [Trading Contracts Demo directory](https://github.com/MystenLabs/sui/examples/trading/contracts/demo)
+The full source code of the demo bear smart contract is available at [Trading Contracts Demo directory](https://github.com/MystenLabs/sui/tree/main/examples/trading/contracts/demo)
 
 :::
 

docs/content/guides/developer/app-examples/trading/indexer-api.mdx

@@ -21,7 +21,7 @@ Before getting started, make sure you:
 
 - Understand [the mechanism behind the Trading backend](../trading.mdx).
 - Install [`pnpm` through this guide](https://pnpm.io/installation) as this example uses it as the package manager.
-- Check out the [indexer's README](https://github.com/MystenLabs/sui/examples/trading/api) to setup the development environment.
+- Check out the [indexer's README](https://github.com/MystenLabs/sui/tree/main/examples/trading/api) to setup the development environment.
 - Check out [Prisma](https://www.prisma.io/) to get an overall sense of the technology that facilitates all the database interactions.
 - Check out [Express](https://expressjs.com/) to learn how to set up a web server application in Node.js. This server bootstraps your API service.
 - Check out [Sui TypeScript SDK](https://sdk.mystenlabs.com/typescript) for basic usage on how to interact with Sui with TypeScript.