Fix INTERVAL sort order doc to be undefined (#222)

diff --git a/src/main/thrift/parquet.thrift b/src/main/thrift/parquet.thrift
index 9decb5a..2084ac6 100644
--- a/src/main/thrift/parquet.thrift
+++ b/src/main/thrift/parquet.thrift
@@ -945,7 +945,7 @@
    *   TIME_MICROS - signed comparison
    *   TIMESTAMP_MILLIS - signed comparison
    *   TIMESTAMP_MICROS - signed comparison
-   *   INTERVAL - unsigned comparison
+   *   INTERVAL - undefined
    *   JSON - unsigned byte-wise comparison
    *   BSON - unsigned byte-wise comparison
    *   ENUM - unsigned byte-wise comparison