SPARK-4971: Fix typo in BlockGenerator comment

Author: CodingCat <zhunansjtu@gmail.com>

Closes #3807 from CodingCat/new_branch and squashes the following commits:

5167f01 [CodingCat] fix typo in the comment

(cherry picked from commit fda4331d58aae454a2b0f4c757d105f8bc228a3d)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>