source-dependencies/value-class-underlying: fix test

We were not testing what we thought we were testing before, because C
used the name "x" which is also the name of the parameter we change in
A, so when we changed A we were invalidating C too. Fixed by using a
different name than "x" in C, which reveals that there is a bug
somewhere since the test doesn't pass anymore.

zinc/src/sbt-test/source-dependencies/value-class-underlying/C.scala

@@ -1,6 +1,6 @@
 object C {
   def main(args: Array[String]): Unit = {
-    val x = B.foo
-    println("x: " + x) // Need to use x in an expression to see if it crashes or not
+    val duck = B.foo
+    println("duck: " + duck) // Need to use duck in an expression to see if it crashes or not
   }
 }