fix typo in doc

doc/tutorial/loop.txt

@@ -244,7 +244,7 @@ Note that we need to iterate over the indices of ``y`` and not over the elements
   accumulator(5)
   k.get_value()
 
-**Scan Example: Computing tanh(v.dot(W) + b) * d where b is binomial**
+**Scan Example: Computing tanh(v.dot(W) + b) * d where d is binomial**
 
 .. code-block:: python