[SPARK-19550] Follow-up: fixed a typo that fails the dev/make-distribution.sh script.

## 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.

dev/make-distribution.sh

@@ -146,7 +146,7 @@ fi
 # Build uber fat JAR
 cd "$SPARK_HOME"
 
-export MAVEN_OPTS="${MAVEN_OPTS:-Xmx2g -XX:ReservedCodeCacheSize=512m}"
+export MAVEN_OPTS="${MAVEN_OPTS:--Xmx2g -XX:ReservedCodeCacheSize=512m}"
 
 # Store the command as an array because $MVN variable might have spaces in it.
 # Normal quoting tricks don't work.