docs: fixing the json response in documentation

Signed-off-by: Vincenzo Palazzo <vincenzopalazzodev@gmail.com>

doc/developers-guide/plugin-development/bitcoin-backend.md

@@ -28,12 +28,12 @@ The plugin must return `feerate_floor` (e.g. 1000 if mempool is empty), and an a
 ```
 {
 	"feerate_floor": <sat per kVB>,
-	"feerates": {
+	"feerates": [
 		{ "blocks": 2, "feerate": <sat per kVB> },
 		{ "blocks": 6, "feerate": <sat per kVB> },
 		{ "blocks": 12, "feerate": <sat per kVB> }
 		{ "blocks": 100, "feerate": <sat per kVB> }
-	}
+	]
 }
 ```
 
@@ -66,4 +66,4 @@ and a boolean `allowhighfees`, which if set means suppress any high-fees check i
 
 The plugin must broadcast it and respond with the following fields:  
     - `success` (boolean), which is `true` if the broadcast succeeded  
-    - `errmsg` (string), if success is `false`, the reason why it failed
+    - `errmsg` (string), if success is `false`, the reason why it failed