Correct the default write buffer size of java doc (#5670)

Summary:
The actual value of default write buffer size within `rocksdb/include/rocksdb/options.h` is 64 MB, we should correct this value in java doc.
Pull Request resolved: https://github.com/facebook/rocksdb/pull/5670

Differential Revision: D16668815

Pulled By: maysamyabandeh

fbshipit-source-id: cc3a981c9f1c2cd4a8392b0ed5f1fd0a2d729afb

java/src/main/java/org/rocksdb/MutableColumnFamilyOptionsInterface.java

@@ -21,7 +21,7 @@ public interface MutableColumnFamilyOptionsInterface
    * Also, a larger write buffer will result in a longer recovery time
    * the next time the database is opened.
    *
-   * Default: 4MB
+   * Default: 64MB
    * @param writeBufferSize the size of write buffer.
    * @return the instance of the current object.
    * @throws java.lang.IllegalArgumentException thrown on 32-Bit platforms