[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