refine CSP doc (#8182)

doc/design/csp.md

@@ -144,7 +144,8 @@ ch = fluid.make_channel(dtype=INT, buffer_size)
 # Now write three elements to the channel
 with fluid.while(steps=buffer_size):
   fluid.send(ch, step)
-  fluid.close_channel(ch)
+
+fluid.close_channel(ch)
 
 with fluid.while(steps=buffer_size):
   fluid.print(fluid.recv(ch))