fix an error in cookbook

Author: wang0618

docs/cookbook.rst

@@ -211,11 +211,12 @@ The following code shows how to redirect stdout of python code and subprocess to
         process = subprocess.Popen("ls -ahl", shell=True, stdout=subprocess.PIPE, stderr=subprocess.STDOUT)
         while True:
             output = process.stdout.readline()
-            if output == '' and process.poll() is not None:
-                break
             if output:
                 put_text(output.decode('utf8'), inline=True)
 
+            if not output and process.poll() is not None:
+                break
+
 
 Add missing syntax highlight for code output
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^