[SPARK-11169] Remove the extra spaces in merge script

[rxin]

Our merge script now turns

[SPARK-1234][SPARK-1235][SPARK-1236][SQL] description

into

[SPARK-1234] [SPARK-1235] [SPARK-1236] [SQL] description

The extra spaces are more annoying in git since the first line of a git commit is supposed to be very short.

Doctest passes with the following command:

python -m doctest merge_spark_pr.py

[SparkQA]

Test build #43886 has finished for PR 9156 at commit ccf9e54.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds no public classes.

[JoshRosen]

Jenkins, retest this please.

[SparkQA]

Test build #43891 has finished for PR 9156 at commit ccf9e54.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[JoshRosen]

LGTM.