use correct branch for git clone (source ref)

Makefile

@@ -48,7 +48,7 @@ ${DEB} deb:
 	install -m 0755 debian/prerm dest/DEBIAN
 	install -m 0755 debian/postrm dest/DEBIAN
 	install -m 0644 debian/triggers dest/DEBIAN
-	echo "git clone git://git.proxmox.com/git/pve-manager.git\\ngit checkout ${GITVERSION}" >  dest/usr/share/doc/${PACKAGE}/SOURCE
+	echo "git clone git://git.proxmox.com/git/pve-manager.git -b stable-3\\ngit checkout ${GITVERSION}" >  dest/usr/share/doc/${PACKAGE}/SOURCE
 	gzip --best dest/usr/share/man/*/*
 	gzip --best dest/usr/share/doc/${PACKAGE}/changelog.Debian
 	dpkg-deb --build dest