fixed typo, test=develop

python/paddle/fluid/layers/learning_rate_scheduler.py

@@ -313,10 +313,12 @@ def cosine_decay(learning_rate, step_each_epoch, epochs):
     """
     Applies cosine decay to the learning rate.
 
-    when training a model, it is oftem recommended to lower the learning rate as the
+    when training a model, it is often recommended to lower the learning rate as the
     training progresses. By using this function, the learning rate will be decayed by
     following cosine decay strategy.
 
+    decayed_lr = learning_rate * 0.5 * (math.cos(epoch * math.pi / epochs) + 1)
+    
     Args:
         learning_rate(Variable|float): The initial learning rate.
         step_each_epoch(int): the number of steps in an epoch.