docs: fix simple typo, exluding -> excluding

There is a small typo in README.rst.

Should read `excluding` rather than `exluding`.

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

Author: timgates42

README.rst

@@ -155,7 +155,7 @@ on a model instance with a protected FSMField will cause an exception.
 
 You can use ``*`` for ``source`` to allow switching to ``target`` from any state. 
 
-You can use ``+`` for ``source`` to allow switching to ``target`` from any state exluding ``target`` state.
+You can use ``+`` for ``source`` to allow switching to ``target`` from any state excluding ``target`` state.
 
 ``target`` state
 ~~~~~~~~~~~~~~~~