MINOR: Update scala default version in readme (apache#9260)

Now, default should be 2.13.x, not 2.12.x. Update it.

Reviewers: Ismael Juma <[email protected]>

README.md

@@ -85,7 +85,7 @@ fail due to code changes. You can just run:
     ./gradlew clean
 
 ### Running a task with one of the Scala versions available (2.12.x or 2.13.x) ###
-*Note that if building the jars with a version other than 2.12.x, you need to set the `SCALA_VERSION` variable or change it in `bin/kafka-run-class.sh` to run the quick start.*
+*Note that if building the jars with a version other than 2.13.x, you need to set the `SCALA_VERSION` variable or change it in `bin/kafka-run-class.sh` to run the quick start.*
 
 You can pass either the major version (eg 2.12) or the full version (eg 2.12.7):