Fix comment: The 6551 baud rate is in the lower *four* bits.

The code is correct, the comment is wrong.

src/main/java/com/loomcom/symon/devices/Acia6551.java

@@ -116,7 +116,7 @@ public class Acia6551 extends Acia {
         if (data == 0) {
             reset();
         } else {
-            // Mask the lower three bits to get the baud rate.
+            // Mask the lower four bits to get the baud rate.
             int baudSelector = data & 0x0f;
             switch (baudSelector) {
                 case 0: