Update custom-indexer.mdx (#20717)

Typo in the docs (missing ".com")

docs/content/guides/developer/advanced/custom-indexer.mdx

@@ -37,7 +37,7 @@ The most straightforward stream source is to subscribe to a remote store of chec
 - Testnet: `https://storage.googleapis.com/mysten-testnet-checkpoints`
 - Mainnet: `https://storage.googleapis.com/mysten-mainnet-checkpoints`
 
-The checkpoint files are stored in the following format: `https://storage.googleapis.com/mysten-testnet-checkpoints/<checkpoint_id>.chk`. You can download the checkpoint file by sending an HTTP GET request to the relevant URL. Try it yourself for checkpoint 1 at [https://storage.googleapis/mysten-testnet-checkpoints/1.chk](https://storage.googleapis/mysten-testnet-checkpoints/1.chk).
+The checkpoint files are stored in the following format: `https://storage.googleapis.com/mysten-testnet-checkpoints/<checkpoint_id>.chk`. You can download the checkpoint file by sending an HTTP GET request to the relevant URL. Try it yourself for checkpoint 1 at [https://storage.googleapis.com/mysten-testnet-checkpoints/1.chk](https://storage.googleapis.com/mysten-testnet-checkpoints/1.chk).
 
 ```mermaid
 flowchart LR