MINOR: README.md manual is changed: aggregatedJavadoc task is extended with a --no-parallel switch (#20683)

Prologue:
#19513 (comment)

Also related to: #20652

@chia7712 I double-checked custom gradle commands in `README.md` and
AFAIK we should be fine now.

Reviewers: Chia-Ping Tsai <[email protected]>

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