Fix markdown for code sample

Author: SamuelMarks

README.rst

@@ -261,6 +261,8 @@ The following actions are available out of the box. They may be used in either
     As ``Store``, but wrap value with ``Some`` for use with ``Option``. For
     example:
 
+.. code::Rust
+
         let mut x: Option<i32> = None;
         ap.refer(&mut x).add_option(&["-x"], StoreOption, "Set var x");