Adds forgotten type annotation

tracim/tracim/controllers/content.py

             # INFO - A.P - Silently intercepts preview exception
             # As preview generation isn't mandatory, just register it
             logger.debug(self, 'Exception: {}'.format(e.__str__))
-        pdf_available = str(enable_pdf_buttons).lower()
+        pdf_available = str(enable_pdf_buttons).lower()  # type: str
 
         fake_api_breadcrumb = self.get_breadcrumb(file_id)
         fake_api_content = DictLikeClass(breadcrumb=fake_api_breadcrumb,