Fix docstring

* lisp/org-beamer.el (org-beamer-environments-extra): Fix docstring

lisp/org-beamer.el

@@ -139,6 +139,7 @@ open    The opening template for the environment, with the following escapes
         %h   the headline text
         %H   if there is headline text, that text in {} braces
         %U   if there is headline text, that text in [] brackets
+        %x   the content of the BEAMER_extra property
 close   The closing string of the environment."
 
   :group 'org-beamer