doc: fix typo. (#594)

Author: jeffFranklin

pytest_django/fixtures.py

@@ -288,7 +288,7 @@ def live_server(request):
     --liveserver command line option or if this is not provided from
     the DJANGO_LIVE_TEST_SERVER_ADDRESS environment variable.  If
     neither is provided ``localhost:8081,8100-8200`` is used.  See the
-    Django documentation for it's full syntax.
+    Django documentation for its full syntax.
 
     NOTE: If the live server needs database access to handle a request
           your test will have to request database access.  Furthermore