Fix documentation discrepancy (#4202)

Change overwrite example to 'true'

rewrite-gradle/src/main/java/org/openrewrite/gradle/AddProperty.java

@@ -45,7 +45,7 @@ public class AddProperty extends ScanningRecipe<AddProperty.NeedsProperty> {
 
     @Option(displayName = "Overwrite if exists",
             description = "If a property with the same key exists, overwrite.",
-            example = "Enable the Gradle build cache")
+            example = "true")
     @Nullable
     Boolean overwrite;