## What changes were proposed in this pull request?

Fixed a typo in `dev/make-distribution.sh` script that sets the MAVEN_OPTS variable, introduced [here](https://github.com/apache/spark/commit/0e24054#diff-ba2c046d92a1d2b5b417788bfb5cb5f8R149).

## How was this patch tested?

Run `dev/make-distribution.sh` manually.

Author: Shuai Lin <linshuai2012@gmail.com>

Closes #16984 from lins05/fix-spark-make-distribution-after-removing-java7.