Fix typos in comments

requirements.txt

 # The base set of requirements for REST framework is actually
 # just Django, but for the purposes of development and testing
-# there are a number of packages that it is useful to install.
+# there are a number of packages that are useful to install.
 
 # Laying these out as seperate requirements files, allows us to
 # only included the relevent sets when running tox, and ensures
-# we are only ever declaring out dependancies in one place.
+# we are only ever declaring our dependencies in one place.
 
 -r requirements/requirements-optionals.txt
 -r requirements/requirements-testing.txt