Use snippet anchor instead of line range

snippets/rust/src/send_payment.rs

@@ -4,8 +4,10 @@ use anyhow::Result;
 use breez_sdk_core::*;
 
 async fn send_payment(sdk: Arc<BreezServices>) -> Result<()> {
+    // ANCHOR: send-payment
     let bolt11 = "...";
     sdk.send_payment(bolt11.into(), None).await?;
+    // ANCHOR_END: send-payment
 
     Ok(())
 }