site: fix slug for release-automation page (#564)

Sadly `updating-plugins` was reused in two pages (this + `plugin-updates.md`).
I linked to this page from krew-index PRs several times.
But now we have to break that one in favor of this and have that link go to
the intended page in the first place.

Signed-off-by: Ahmet Alp Balkan <ahmetb@google.com>

site/content/docs/developer-guide/release/release-automation.md

@@ -1,6 +1,6 @@
 ---
 title: Automating plugin updates
-slug: updating-plugins
+slug: automating-updates
 weight: 300
 ---