Fix docstrings to use "UTF-8" instead of "utf8"

The UTF-8 FAQ says this is the correct name for the encoding.

  http://www.cl.cam.ac.uk/~mgk25/unicode.html#utf-8

  The official name and spelling of this encoding is UTF-8, where UTF
  stands for UCS Transformation Format. Please do not write UTF-8 in
  any documentation text in other ways (such as utf8 or UTF_8), unless
  of course you refer to a variable name and not the encoding itself.

test/test_exceptions.py

@@ -57,7 +57,7 @@ class ExceptionsTest(TemplateTest):
 
     @requires_pygments_14
     def test_utf8_html_error_template_pygments(self):
-        """test the html_error_template with a Template containing utf8
+        """test the html_error_template with a Template containing UTF-8
         chars"""
 
         if compat.py3k:
@@ -105,7 +105,7 @@ ${u'привет'}
 
     @requires_no_pygments_exceptions
     def test_utf8_html_error_template_no_pygments(self):
-        """test the html_error_template with a Template containing utf8
+        """test the html_error_template with a Template containing UTF-8
         chars"""
 
         if compat.py3k: