[SPARK-3372] [MLlib] MLlib doesn't pass maven build / checkstyle due to multi-byte character contained in Gradient.scala

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2248 from sarutak/SPARK-3372 and squashes the following commits:

73a28b8 [Kousuke Saruta] Replaced UTF-8 hyphen with ascii hyphen

(cherry picked from commit 1bed0a3869a526241381d2a74ba064e5b3721336)
Signed-off-by: Xiangrui Meng <meng@databricks.com>