Use snippet anchor instead of line range

snippets/rust/src/buy_btc.rs

@@ -4,11 +4,13 @@ use anyhow::Result;
 use breez_sdk_core::*;
 
 async fn buy(sdk: Arc<BreezServices>) -> Result<()> {
+    // ANCHOR: buy-btc
     let res = sdk.buy_bitcoin(
         BuyBitcoinRequest {
             provider: BuyBitcoinProvider::Moonpay,
             opening_fee_params: None})
         .await?;
+    // ANCHOR_END: buy-btc
 
     Ok(())
 }