Fixed field data regex filter settings

Setting regex filter as string caused `Failed to get setting group for [filter.] setting prefix and setting [filter.regex] because of a missing '.'` error.

According to https://github.com/elasticsearch/elasticsearch/blob/master/src/main/java/org/elasticsearch/index/fielddata/plain/AbstractBytesIndexFieldData.java#L129 it should be set as `pattern`.

Using `pattern` setting key isn't throwing error anymore and filtering is working fine.

//cc: @s1monw