[SPARK-11388][BUILD] Fix self closing tags.

Java 8 javadoc does not like self closing tags: ```<p/>```, ```<br/>```, ...

This PR fixes those.

Author: Herman van Hovell <[email protected]>

Closes #9339 from hvanhovell/SPARK-11388.

Author: hvanhovell

launcher/src/main/java/org/apache/spark/launcher/SparkAppHandle.java

@@ -19,7 +19,7 @@
 
 /**
  * A handle to a running Spark application.
- * <p/>
+ * <p>
  * Provides runtime information about the underlying Spark application, and actions to control it.
  *
  * @since 1.6.0
@@ -110,7 +110,7 @@ public interface Listener {
      * Callback for changes in the handle's state.
      *
      * @param handle The updated handle.
-     * @see {@link SparkAppHandle#getState()}
+     * @see SparkAppHandle#getState()
      */
     void stateChanged(SparkAppHandle handle);
 

launcher/src/main/java/org/apache/spark/launcher/SparkLauncher.java

@@ -350,7 +350,7 @@ public SparkLauncher setVerbose(boolean verbose) {
 
   /**
    * Launches a sub-process that will start the configured Spark application.
-   * <p/>
+   * <p>
    * The {@link #startApplication(SparkAppHandle.Listener...)} method is preferred when launching
    * Spark, since it provides better control of the child application.
    *
@@ -362,16 +362,16 @@ public Process launch() throws IOException {
 
   /**
    * Starts a Spark application.
-   * <p/>
+   * <p>
    * This method returns a handle that provides information about the running application and can
    * be used to do basic interaction with it.
-   * <p/>
+   * <p>
    * The returned handle assumes that the application will instantiate a single SparkContext
    * during its lifetime. Once that context reports a final state (one that indicates the
    * SparkContext has stopped), the handle will not perform new state transitions, so anything
    * that happens after that cannot be monitored. If the underlying application is launched as
    * a child process, {@link SparkAppHandle#kill()} can still be used to kill the child process.
-   * <p/>
+   * <p>
    * Currently, all applications are launched as child processes. The child's stdout and stderr
    * are merged and written to a logger (see <code>java.util.logging</code>). The logger's name
    * can be defined by setting {@link #CHILD_PROCESS_LOGGER_NAME} in the app's configuration. If