Update README and test

exercises/diamond/README.md

 ··A··
 ```
 
-Diamond for letter 'E':
+Diamond for letter 'D':
 
 ```text
-····A····
-···B·B···
-··C···C··
-·D·····D·
-E·······E
-·D·····D·
-··C···C··
-···B·B···
-····A····
+···A···
+··B·B··
+·C···C·
+D·····D
+·C···C·
+··B·B··
+···A···
 ```
 
 # Running the tests

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

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