fix errors

python/paddle/fluid/io.py

@@ -887,7 +887,7 @@ def load_checkpoint(executor, checkpoint_dir, serial, main_program):
 
     In the training precess, we generally save a checkpoint in each
     iteration. So there are more than one checkpoint in the 
-    `checkpoint_dir`(each checkpoint has its own sub folder), use
+    `checkpoint_dir` (each checkpoint has its own sub folder), use 
     `serial` to specify which serial of checkpoint you would like to
     load.