Wish: Add graphviz feature usage to examples/ex1.rs

[stappersg]

My wish expressed as a patch:

--- a/examples/ex1.rs
+++ b/examples/ex1.rs
@@ -2,6 +2,9 @@
 //!
 //! A simple example of a state machine which will get stuck in the final state.
 //! A picture depicting the state machine can be found in the README.
+//!
+//! to generate that picture do
+//!   https://github.com/korken89/smlang-rs/issues/42
 
 #![deny(missing_docs)]
 

So a change in examples/ex1.rs, not necessary something that is active, having it as comment is a improvement.

There is no need for changes that will effect the ci build system requiring graphviz being installed.
Just document how to use the graphviz feature.