change 'that' to 'self'

Author: Chris Anderson

doc/en/function/this.md

@@ -72,14 +72,14 @@ In order to gain access to `Foo` from within `test`, it is necessary to create a
 local variable inside of `method` that refers to `Foo`.
 
     Foo.method = function() {
-        var that = this;
+        var self = this;
         function test() {
-            // Use that instead of this here
+            // Use self instead of this here
         }
         test();
     }
 
-`that` is just a normal variable name, but it is commonly used for the reference to an 
+`self` is just a normal variable name, but it is commonly used for the reference to an
 outer `this`. In combination with [closures](#function.closures), it can also 
 be used to pass `this` values around.