Fix checkstyle error

[abhilash1in]

Fix checkstyle error related to variable naming convention:

[INFO] --- checkstyle:3.1.0:check (default-cli) @ zookeeper-jute ---
[INFO] Starting audit...
[ERROR] /Users/abkishor/code/opensource/li-zookeeper/zookeeper-jute/src/main/java/org/apache/jute/BinaryOutputArchive.java:111:13: Name 'length_descriptor_size' must match pattern '^[a-z][a-zA-Z0-9]*$'. [LocalVariableName]
Audit done.
[INFO] There is 1 error reported by Checkstyle 8.19 with checkstyle-strict.xml ruleset.
[ERROR] src/main/java/org/apache/jute/BinaryOutputArchive.java:[111,13] (naming) LocalVariableName: Name 'length_descriptor_size' must match pattern '^[a-z][a-zA-Z0-9]*$'.

length_descriptor_size was added in #118