Fix a wrong description in the avro-tools random's usage

$ java -jar lang/java/tools/target/avro-tools-1.10.0-SNAPSHOT.jar random
Usage: outFile (filename or '-' for stdout)
Option                  Description                                       
------                  -----------                                       
--codec <String>        Compression codec (default: null)                 

$ java -jar lang/java/tools/target/avro-tools-1.10.0-SNAPSHOT.jar random --count 0 --schema-file share/test/schemas/weather.avsc /tmp/outFile.avro
$ java -jar lang/java/tools/target/avro-tools-1.10.0-SNAPSHOT.jar getmeta /tmp/outFile.avro 
avro.schema	{"type":"record","name":"Weather","namespace":"test","doc":"A weather reading.","fields":[{"name":"station","type":"string","order":"ignore"},{"name":"time","type":"long"},{"name":"temp","type":"int"}]}
avro.codec	deflate