Re-add HTML anchor so existing links to it still work (refs: #15557).

We might have such links e.g. in Help Desk's template answers.

wiki/src/support/known_issues.mdwn

@@ -488,6 +488,7 @@ because the file is corrupt". You can work around this by downloading
 the video and watching it in the Totem video player.
 
 <a id="automatic_upgrade_fails"></a>
+<a id="partial-upgrade"></a>
 
 Tails fails to start or behaves weirdly after an automatic upgrade
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