Update B.2.1. Nested properties.md

X. Appendices/B.2.1. Nested properties.md

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+### 附录 B.2.1 内嵌属性
+
+该注解处理器自动将内部类当做内嵌属性处理。例如，下面的类：
+```java
+@ConfigurationProperties(prefix="server")
+public class ServerProperties {
+
+    private String name;
+
+    private Host host;
+
+    // ... getter and setters
+
+    private static class Host {
+
+        private String ip;
+
+        private int port;
+
+        // ... getter and setters
+
+    }
+
+}
+```