Fix wrong file name pattern in .gitignore

[sarutak]

In .gitignore, there is an entry for spark--bin.tar.gz but considering make-distribution.sh, the name pattern should be spark--bin-*.tgz.

This change is really small so I don't open issue in JIRA. If it's needed, please let me know.

[SparkQA]

Test build #23976 has started for PR 3529 at commit de3c70a.

• This patch merges cleanly.

[rxin]

Thanks. I've merged this in master.

[SparkQA]

Test build #23976 has finished for PR 3529 at commit de3c70a.

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

[AmplabJenkins]

Test PASSed.
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Test PASSed.