Fix checkstyle problems

src/main/java/dan200/computercraft/shared/util/NBTUtil.java

@@ -190,11 +190,11 @@ public static String getNBTHash( @Nullable CompoundNBT tag )
         }
     }
 
-    private static class DigestOutputStream extends OutputStream
+    private static final class DigestOutputStream extends OutputStream
     {
         private final MessageDigest digest;
 
-        private DigestOutputStream( MessageDigest digest )
+        DigestOutputStream( MessageDigest digest )
         {
             this.digest = digest;
         }