Refactor XML output format in sendReadingXML function

src/main.cpp

@@ -44,11 +44,7 @@ void loop()
 
 void sendReadingXML(uint32_t timestamp, float temperature, float humidity, const char *id)
 {
-  Serial.print("<reading time=\"");
-  Serial.print(timestamp);
-  Serial.print("\" id=\"");
-  Serial.print(id);
-  Serial.println("\">");
+  Serial.println("<reading>");
 
   Serial.print("    <temp unit=\"C\">");
   Serial.print(temperature, 2);