clarify file name references in documentation for upgrading from 3.2 to 3.3

The documentation for upgrading from 3.2 to 3.3 seems to loop.

It talks about an installer but links to a download page where there is no installer.

Installing from the main download includes a rather curious instruction to upgrade from the scripts in the /bin without mentioning which script.

I have tagged it as a blocker since one can not upgrade from 3.x to 3.y as near as I can figure out from the pages I found.

https://support.sonatype.com/hc/en-us/articles/217967608-How-to-Upgrade-to-Nexus-Repository-Manager-3

leads to here

https://support.sonatype.com/hc/en-us/articles/115000350007

leads to here

https://support.sonatype.com/hc/en-us/articles/218637467