Fix broken link

[tisonkun]

文件 https://github.com/alibaba/jstorm/blob/master/docs/jstorm-doc/Community/JStormUsers.md 的链接失效，原因是这个 .jpg 文件的后缀名大小写问题，PR 应当使得未能正确显示图片的 http://jstorm.io/Community/JStormUsers.html 页面正确显示图片

[CLAassistant]

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[CLAassistant]

Thank you for your submission, we really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}