Fix typo.

README.rst

@@ -376,7 +376,7 @@ People that helped spotting bugs, providing solutions, etc.:
  - `lieryan <https://github.com/lieryan>`_ brought up the `prefix
    code`_ problem.
  - `LogosOfJ <https://github.com/LogosOfJ>`_ discovered and fixed
-   serious `realdice` source of randomnoess problem.
+   serious `realdice` source of randomness problem.
 
 Many thanks to all of them!