Ch08: Replace concat with append in one example.

Fixes issue #479.

Author: eevman

ch08.md

@@ -67,7 +67,7 @@ Container.of(2).map(two => two + 2);
 Container.of('flamethrowers').map(s => s.toUpperCase()); 
 // Container('FLAMETHROWERS')
 
-Container.of('bombs').map(concat(' away')).map(prop('length')); 
+Container.of('bombs').map(append(' away')).map(prop('length')); 
 // Container(10)
 ```