Suggest updating the Wiki

[jujn]

Maven officials have pointed out that the URL "cental.maven.org" has been abandoned. Please refer to the details（ "https://central.sonatype.org/news/20190715_http_deprecation_update/")。 So, On the page "https://github.com/alibaba/druid/wiki/%E5%B8%B8%E8%A7%81%E9%97%AE%E9%A2%98", "2 Where to download Druid?", the Maven central repository address should be changed to" https://repo1.maven.org/maven2/com/alibaba/druid/".