update the code for Chapel 2.x

Author: Alex Razoumov

episodes/13-synchronization.md

@@ -153,12 +153,12 @@ var x: sync int, a: int;
 writeln("this is main task launching a new task");
 begin {
   for i in 1..10 do writeln("this is new task working: ",i);
-  x = 2;
+  x.writeEF(2);
   writeln("New task finished");
 }
 
 writeln("this is main task after launching new task... I will wait until  it is done");
-a = x;   // don't run this line until the variable x is written in the other task
+a = x.readFF();   // don't run this line until the variable x is written in the other task
 writeln("and now it is done");
 ```
 
@@ -194,6 +194,15 @@ statement?
 
 Discuss your observations.
 
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 :::::::::::::::::::::::::::::::::::::::::::::::::::
 
 There are a number of methods defined for _sync_ variables. Suppose _x_ is a sync variable of a given type,