remove space

doc/tutorial/extending_theano.txt

@@ -137,7 +137,7 @@ An op has to implement some methods defined in the the interface of
       dtype, but can have the wrong shape and have any stride pattern.
 
   :func:`perform` method must be determined by the inputs. That is to say,
-  when applied to similar inputs the method must return the same outputs.
+  when applied to identical inputs the method must return the same outputs.
 
   :class:`gof.Op` allows some other way to define the op implentation.
   For instance, it is possible to define :meth:`Op.c_code` to provide a