Update ci.yml to use javadoc:javadoc

Instead of javadoc:aggregate, to catch issues like 5bf91fb

h/t/ @cpovirk

Author: cushon

.github/workflows/ci.yml

@@ -92,7 +92,7 @@ jobs:
         # only generate javadoc on JDKs with the fix for https://bugs.openjdk.org/browse/JDK-8241780
         if: ${{ matrix.java >= 15 }}
         shell: bash
-        run: mvn -P '!examples' javadoc:aggregate
+        run: mvn -P '!examples' javadoc:javadoc
 
   publish_snapshot:
     name: 'Publish snapshot'