fix in "Layer.compute_output_shape" description
diff --git a/keras/engine/base_layer.py b/keras/engine/base_layer.py
index 0482fe7bb52..3dac15be7e4 100644
--- a/keras/engine/base_layer.py
+++ b/keras/engine/base_layer.py
@@ -580,7 +580,7 @@ def compute_output_shape(self, input_shape):
                 instead of an integer.
 
         # Returns
-            An input shape tuple.
+            An output shape tuple.
         """
         return input_shape
 

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### Summary

### Related Issues
Fixed a logical error in the description of "Layer.compute_output_shape" method
### PR Overview

- [n] This PR requires new unit tests [y/n] (make sure tests are included)
- [n] This PR requires to update the documentation [y/n] (make sure the docs are up-to-date)
- [y] This PR is backwards compatible [y/n]
- [n] This PR changes the current API [y/n] (all API changes need to be approved by fchollet)
