Update PathMappingCache.java

Update a comment

apm-sniffer/apm-sdk-plugin/nutz-plugins/mvc-annotation-1.x-plugin/src/main/java/org/skywalking/apm/plugin/nutz/mvc/PathMappingCache.java

@@ -4,8 +4,9 @@ import java.lang.reflect.Method;
 import java.util.concurrent.ConcurrentHashMap;
 
 /**
- * The <code>PathMappingCache</code> represents a field
- *
+ * The <code>PathMappingCache</code> represents a mapping cache.
+ * key: {@link Method}
+ * value: the url pattern
  *
  * @author wendal
  */