use the right method to emit warnings

Use warn() instead of the non-existant warning() method.

Fixes #5

sphinxcontrib/spelling/builder.py

@@ -148,5 +148,5 @@ def finish(self):
         self.info('Spelling checker messages written to %s' %
                   self.output_filename)
         if self.misspelling_count:
-            self.warning('Found %d misspelled words' % self.misspelling_count)
+            self.warn('Found %d misspelled words' % self.misspelling_count)
         return