Update test case for diamond

exercises/diamond/src/test/java/DiamondPrinterTest.java

 
     @Ignore("Remove to run test")
     @Test
-    public void testFiveByFiveDiamond() {
-        List<String> output = diamondPrinter.printToList('E');
+    public void testFourByFourDiamond() {
+        List<String> output = diamondPrinter.printToList('D');
         assertThat(output, is(asList("    A    ",
                                      "   B B   ",
                                      "  C   C  ",
                                      " D     D ",
-                                     "E       E",
-                                     " D     D ",
                                      "  C   C  ",
                                      "   B B   ",
                                      "    A    ")));