Update scalafmt versions in documentation

Author: mdedetrich

plugin-gradle/README.md

@@ -389,7 +389,7 @@ spotless {
 spotless {
   scala {
     // version and configFile, majorScalaVersion are all optional
-    scalafmt('2.6.1').configFile('scalafmt.conf').majorScalaVersion('2.13')
+    scalafmt('3.5.9').configFile('scalafmt.conf').majorScalaVersion('2.13')
 ```
 
 <a name="applying-to-cc-sources"></a>

plugin-maven/README.md

@@ -393,7 +393,7 @@ Groovy-Eclipse formatting errors/warnings lead per default to a build failure. T
 
 ```xml
 <scalafmt>
-  <version>2.0.1</version>              <!-- optional -->
+  <version>3.5.9</version>              <!-- optional -->
   <file>${project.basedir}/scalafmt.conf</file> <!-- optional -->
   <majorScalaVersion>2.13</majorScalaVersion> <!-- optional -->
 </scalafmt>