MINOR: README.md manual is changed: aggregatedJavadoc task is extended with a --no-parallel switch

Author: dejan2609

README.md

@@ -29,7 +29,7 @@ Follow instructions in https://kafka.apache.org/quickstart
     ./gradlew srcJar
 
 ### Build aggregated javadoc ###
-    ./gradlew aggregatedJavadoc
+    ./gradlew aggregatedJavadoc --no-parallel
 
 ### Build javadoc and scaladoc ###
     ./gradlew javadoc