releng: Update steps to build javadoc in README.md

Change-Id: I1bdbdbff369305b0cb4b1f4ab22cab1505ae10f3
Signed-off-by: Bernd Hufmann <bernd.hufmann@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/c/tracecompass/org.eclipse.tracecompass/+/205639
Tested-by: Trace Compass Bot <tracecompass-bot@eclipse.org>
Tested-by: Patrick Tasse <patrick.tasse@gmail.com>
Reviewed-by: Patrick Tasse <patrick.tasse@gmail.com>

README.md

@@ -55,7 +55,7 @@ placed in `releng/org.eclipse.tracecompass.releng-site/target/repository`.
 To generate the javadoc from the Trace Compass source code, run the following
 command from the top-level directory:
 
-    mvn clean package javadoc:aggregate
+    mvn compile javadoc:aggregate
 
 The javadoc html files will be under `target/site/apidocs`.