[releng] Point at specific p2 repo for platform

This URL is only used for help generation, but as we are trying to build
before 4.21 is released, avoid the composite and point at the direct
repo.

Change-Id: I5ed3cde2231af78ca6a75829bbd2a99f93ed5ed7

pom.xml

diff --git a/pom.xml b/pom.xml
index e49396e..c85aee6 100644
--- a/pom.xml
+++ b/pom.xml
@@ -43,7 +43,7 @@
 
 		<!-- When changing this value, consider also bumping the version numbers of all the docs plug-ins so that compare&replace does not
 		     replace the the newly built docs because the only change is in the filtered out index/* resources. -->
-		<help-docs-eclipserun-repo>http://download.eclipse.org/eclipse/updates/4.21/</help-docs-eclipserun-repo>
+		<help-docs-eclipserun-repo>http://download.eclipse.org/eclipse/updates/4.21/R-4.21-202109060500/</help-docs-eclipserun-repo>
 
 		<tycho.scmUrl>scm:git:https://git.eclipse.org/r/cdt/org.eclipse.cdt</tycho.scmUrl>
 		<base.test.vmargs>-Xms256m -Xmx512m -ea</base.test.vmargs>