chore: replace RPC URLs to our prod endpoints

pages/chain/get-started/rpc.mdx

@@ -22,7 +22,7 @@ As an EVM-compatible chain, you can connect to Beam network using any Ethereum w
 | Beam Mainnet       | Settings                                        |
 | :----------------- | :---------------------------------------------- |
 | Network name       | `Beam Mainnet`                                  |
-| RPC URL            | `https://subnets.avax.network/beam/mainnet/rpc` |
+| RPC URL            | `https://build.onbeam.com/rpc`                  |
 | Chain ID           | `4337`                                          |
 | Currency symbol    | `BEAM`                                          |
 | Block explorer URL | https://subnets.avax.network/beam               |
@@ -61,7 +61,7 @@ Enter the Beam Mainnet settings as provided above.
 | Beam Testnet       | Settings                                        |
 | :----------------- | :---------------------------------------------- |
 | Network name       | `Beam Testnet`                                  |
-| RPC URL            | `https://subnets.avax.network/beam/testnet/rpc` |
+| RPC URL            | `https://build.onbeam.com/testnet/rpc`          |
 | Chain ID           | `13337`                                         |
 | Currency symbol    | `BEAM`                                          |
 | Block explorer URL | https://subnets-test.avax.network/beam          |