docs: fix simple typo, paramter -> parameter

There is a small typo in docs/directives.rst.

Should read `parameter` rather than `paramter`.

Signed-off-by: Tim Gates <tim.gates@iress.com>

docs/directives.rst

@@ -76,7 +76,7 @@ TOC plugin::
     )
 
 If **TOC** directive is enabled, the ``heading`` method of renderer will accept
-one more paramter::
+one more parameter::
 
     def heading(self, text, level):
         # without TOC directive