fixed a typo in the dev_start_guide

doc/extending/op.txt

@@ -133,6 +133,22 @@ following methods:
   forgotten.  Symbolic differentiation will fail on a graph that
   includes this Op.
 
+  It is important to understand that this is not meant to return
+  the gradient of the Op's output but rather the gradient of some
+  other criterion C with respect to the Op's input.
+
+  If the outputs of your op are [ f_1, ... f_n], then
+  ``output_derivatives`` gives [ grad_{f_1} C, grad_{f_2} C, ... , grad_{f_n} C ]
+  If the inputs of your op are [x_1, ..., x_n], then your Op.grad should
+  return [ grad_{x_1} C, grad_{x_2} C, ..., grad_{x_n} C ]
+
+  where (grad_{y} z)_i = partial z / partial y_i  (and i can have any
+  number of dimensions)
+  (note: in the case where i is 2 dimensional, this definition of grad
+  is different from the standard mathematical definition of the gradient
+  of a scalar with respect to a matrix, where you transpose the indices)
+ 
+
 
 At a bare minimum, a new Op must define ``make_node`` and ``perform``, which have no defaults.
 

doc/internal/dev_start_guide.txt

@@ -39,7 +39,7 @@ you should clone this repository like this:
 
 .. code-block:: bash
 
-    hg clone 'http://username:password@hg.assembla.com/theano Theano'
+    hg clone 'http://username:password@hg.assembla.com/theano' Theano
 
 You can also clone the code anonymously: