Update example pre-publish-send-email hook script

The naming prefixes has changed.  This patch has no impact since this
script is just an example, but keep it up-to-date for the sake of
completeness.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>

hooks/pre-publish-send-email.example

@@ -28,4 +28,4 @@ check '^Subject: \[.*RH.*\]' 'missing RHEL/RHEV/RHV tag in Subject: line'
 check '^Subject: \[.*qemu-kvm.*\]' 'missing qemu-kvm/qemu-kvm-rhev tag in Subject: line'
 check '^\(Bugzilla\|BZ\): ' 'missing Bugzilla: header in cover letter'
 check '^\(Brew\|BREW\): ' 'missing Brew: header in cover letter'
-check '^Upstream: ' 'missing Upstream: header in cover letter'
+check '^\(Upstream\|UPSTREAM\): ' 'missing Upstream: header in cover letter'