Correct references to the wrong exercise in chapter 19

In several exercises, the comments were referring to the exercise as one below
its actual index:
 * exercise 4 introduced itself as exercise 3,
 * exercise 5 introduced itself as exercise 4,
 * etc.

Author: multimentha

chp19_data_streams/exercise_19_04_clientEvent/exercise_19_04_clientEvent.pde

@@ -2,7 +2,7 @@
 // Daniel Shiffman
 // http://www.learningprocessing.com
 
-// Exercise 19-3: Go back and rewrite the therapy client
+// Exercise 19-4: Go back and rewrite the therapy client
 // to use <code>clientEvent()</code> rather than 
 // client.available() in draw(). This is trickier than 
 // it might initially seem because it requires the use 

chp19_data_streams/exercise_19_05_whiteboard_color/exercise_19_05_whiteboard_color.pde

@@ -2,7 +2,7 @@
 // Daniel Shiffman
 // http://www.learningprocessing.com
 
-// Exercise 19-4: Expand the whiteboard to allow for 
+// Exercise 19-5: Expand the whiteboard to allow for 
 // color. Each client should send a red, green, and blue 
 // value in addition to the XY location. You will not 
 // need to make any changes to the server for this to work.

chp19_data_streams/exercise_19_06_pong/exercise_19_06_pong.pde

@@ -2,7 +2,7 @@
 // Daniel Shiffman
 // http://www.learningprocessing.com
 
-// Exercise 19-5: Create a two-player game of Pong 
+// Exercise 19-6: Create a two-player game of Pong 
 // played over the network. This is a complex assignment, 
 // so build it up slowly. For instance, you should get 
 // Pong to work first without networking (if you are 

chp19_data_streams/exercise_19_07_arduino/exercise_19_07_arduino.pde

@@ -2,7 +2,7 @@
 // Daniel Shiffman
 // http://www.learningprocessing.com
 
-// Exercise 19-6: If you have an Arduino board, build 
+// Exercise 19-7: If you have an Arduino board, build 
 // your own interface to control a Processing sketch 
 // you have already made. (Before you attempt this, 
 // you should make sure you can successfully run the