Fix two javadoc

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/AbstractTracerContext.java

@@ -103,6 +103,8 @@ public interface AbstractTracerContext {
 
     /**
      * Notify this context, current span is going to be finished async in another thread.
+     *
+     * @return The current context
      */
     AbstractTracerContext awaitFinishAsync();
 

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/context/AsyncSpan.java

@@ -32,7 +32,8 @@ public interface AsyncSpan {
      * The span finish at current tracing context, but the current span is still alive, until {@link #asyncFinish}
      * called.
      *
-     * This method must be called<br/>
+     * This method must be called
+     * 
      * 1. In original thread(tracing context).
      * 2. Current span is active span.
      *