remove Non-ASCII character '\xc2'

python/paddle/fluid/layers/control_flow.py

@@ -252,7 +252,7 @@ class ParallelDo(object):
         ||||   Compute backward pass in parallel
         |      accumulate param@grad from different devices to the first device
         |      Merge input@grad from different devices
-        |      Copy param@grad to the place of parallel_do_op
+        |      Copy param@grad to the place of parallel_do_op
 
     Examples: