Fix some typos (pytorch#111)

* Fix small typo

* Fix a sentence in exercises

* Fix some typos

advanced_source/neural_style_tutorial.py

@@ -14,7 +14,7 @@
 Neural what?
 ~~~~~~~~~~~~
 
-The Neural-Style, or Neural-Transfer, is an algorithm that taks as
+The Neural-Style, or Neural-Transfer, is an algorithm that takes as
 input a content-image (e.g. a tortle), a style-image (e.g. artistic
 waves) and return the content of the content-image as if it was
 'painted' using the artistic style of the style-image:
@@ -25,8 +25,8 @@
 How does it work?
 ~~~~~~~~~~~~~~~~~
 
-The principe is simple: we define two distances, one for the content
-(:math:`D_C`) and one for the style (:math:`D_S`). :math:`D_C` measues
+The principle is simple: we define two distances, one for the content
+(:math:`D_C`) and one for the style (:math:`D_S`). :math:`D_C` measures
 how different is the content between two images, while :math:`D_S`
 measures how different is the style between two images. Then, we take a
 third image, the input, (e.g. a with noise), and we transform it in