Clean up 2.5 upgrade instructions

This commit cleans up a few tiny issues and a large formatting bug,
fixes a filename, and reorders the linux distros to put the newest ones
first. The content related changes are:

  - fix filename in 'tar' command
  - reorder linux distros (in two places)
  - fix various punctuation (add comma; remove colon in header)

And the AsciiDoc changes:

  - add asciidoc '+'s before and after 'indexterm' commands to fix
    broken ordered list formatting
  - remove an unnecessary asciidoc '+'
  - correct the length of an asciidoc block header line (doesn't change
    behavior, but helps readability of the text file)

Signed-off-by: Remington Steed <rjs7@calvin.edu>