Correct javadoc for Env#setBackgroundThreads(int) (facebook#8576)

Summary:
By default, the low priority pool is not the flush pool, so calling `Env#setBackgroundThreads` without providing a priority will not do what the caller expected.

Pull Request resolved: facebook#8576

Reviewed By: ajkr

Differential Revision: D29925154

Pulled By: mrambacher

fbshipit-source-id: cd7211fc374e7d9929a9b88ea0a5ba8134b76099

java/src/main/java/org/rocksdb/Env.java

@@ -43,8 +43,8 @@ public static Env getDefault() {
   }
 
   /**
-   * <p>Sets the number of background worker threads of the flush pool
-   * for this environment.</p>
+   * <p>Sets the number of background worker threads of the low priority
+   * pool for this environment.</p>
    * <p>Default number: 1</p>
    *
    * @param number the number of threads