ZEO: Fix ZEO4-wc2 installation

- due to a thinko in c170fe3e (ZODB: Provide ZODB4-wc2 which backports MVCC approach from ZODB5)
  4-wc2 revision was set on a ZEO(egg) section instead of ZEO(repository)
  section. As the result it was ZEO4 - not ZEO4-wc2 - that was installed.

  This was leading to hangs in tests with wcfs waiting for ZODB server to
  update it after a commit, but no notification was coming from ZEO at all.

  For the refrence: the reason ZEO4-wc2 exists at all is the backport patches
  that teach ZEO to send invalidations even on empty transactions
  (nexedi/ZEO@bf80d23d), and to include
  just-created objects into invalidations as well (nexedi/ZEO@f2fae122).
  Those changes are needed for WCFS to be always notificated on every
  transaction and with full set of changed objects.

- there was a typo: it was `revisions` instead of `revision`.

-> Fix both issues. Now it is ZEO4-wc2 that is installed for real when `ZODB.major = 4-wc2`.

component/ZEO/buildout.cfg

@@ -76,5 +76,9 @@ egg-versions =
 # See https://github.com/zopefoundation/ZEO/pull/161 for the reference.
 [ZEO4-wc2]
 <= ZEO4
-revisions = bf80d23d3506
+setup   = ${ZEO4-wc2-repository:location}
+
+[ZEO4-wc2-repository]
+<= ZEO4-repository
+revision = bf80d23d3506
 location = ${buildout:parts-directory}/ZEO4-wc2