[PATCH] clean up release notes

Hi,

following patch fixes some more W3C validation issues inside the release
notes

[[[
Make the release notes more compliant with the XHTML 1.1 standard.

* docs/release-notes/1.6.html
  (Testsuite fails): add missing closing p-tag

* docs/release-notes/1.7.html
  (svn diff can show git-style diff annotations): close p-tag before ul

* docs/release-notes/1.8.html
  (Skelta style updates are now the default): drop invalid p-section

* docs/release-notes/1.9.html
  (fsvnadmin pack): correct typo (missing / for closing tag)
  (svnadmin setrevprop --transaction): fix incorrect closing tag
]]]

Regards,
Stefan