Update FindCallGraphTest to expect void for Kotlin Unit return type (#43)

Non-nullable kotlin.Unit return types now map to JVM void as of
rewrite 8.79.4 (openrewrite/rewrite#7364). Update the call graph
assertion to match.

Author: timtebeek

src/test/java/org/openrewrite/FindCallGraphTest.java

@@ -382,7 +382,7 @@ void companionObject() {
                 "println",
                 "kotlin.Any",
                 CallGraph.ResourceType.METHOD,
-                "kotlin.Unit"
+                "void"
               )
             )
           ),