Update Graphiti 0.18.0 contribution for RC2

Change-Id: Ida4c3aff0ab2b9a0409a04227eb09c8324b5e537
diff --git a/gmp-graphiti.aggrcon b/gmp-graphiti.aggrcon
index e1bff05..5f4ddcd 100644
--- a/gmp-graphiti.aggrcon
+++ b/gmp-graphiti.aggrcon
@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="ASCII"?>
 <aggregator:Contribution xmi:version="2.0" xmlns:xmi="http://www.omg.org/XMI" xmlns:aggregator="http://www.eclipse.org/cbi/p2repo/2011/aggregator/1.1.0" description="The Eclipse Modeling Graphiti project" label="Graphiti for SimRel 2021-06">
-  <repositories location="https://download.eclipse.org/graphiti/updates/milestones/S-0.18.0M3-202105181817/" description="Graphiti 0.18.0 Milestones Update Site">
+  <repositories location="https://download.eclipse.org/graphiti/updates/milestones/S-0.18.0RC2-202106091000/" description="Graphiti 0.18.0 Milestones Update Site">
     <features description="The feature containing the Graphiti framework" name="org.eclipse.graphiti.feature.feature.group" versionRange="[0.18.0,0.19.0)">
       <categories href="simrel.aggr#//@customCategories[identifier='Modeling']"/>
     </features>