fix typo following code review.

NEWS.txt

@@ -3,7 +3,7 @@
 Update in the Trunk since the last release:
 
 * Added in the tutorial documentation on how to extend Theano.
-  This explain how to make a Theano Op from a python function.
+  This explains how to make a Theano Op from a Python function.
   http://deeplearning.net/software/theano/tutorial/extending_theano.html
 
 

doc/tutorial/extending_theano.txt

@@ -215,7 +215,7 @@ GPU Op
 ------
 
 Op that execute on the GPU should inherit from the
-``theano.sandbox.cuda.GpuOp`` and not ``theano.Op``. This allow Theano
+``theano.sandbox.cuda.GpuOp`` and not ``theano.Op``. This allows Theano
 to make the distinction between both. Currently, we use this to test
-if the NVIDIA driver work correctly with our sum reduction code on the
+if the NVIDIA driver works correctly with our sum reduction code on the
 gpu.