Typo in example of README

[marcofavorito]

Describe the bug
In the "How to use" section of the README, the example is not correct (README at the commit: https://github.com/whitemech/pythomata/blob/cb521b8c98a63069ebc7a36e3ac43d47ab2941e6/README.md).

By replacing the transition:

(s2, a, s3)

With the transition:

(s2, c, s3)

It works again.

The code snippet becomes:

from pythomata import SimpleDFA
alphabet = {"a", "b", "c"}
states = {"s1", "s2", "s3"}
initial_state = "s1"
accepting_states = {"s3"}
transition_function = {
    "s1": {
        "b" : "s1",
        "a" : "s2"
    },
    "s2": {
        "c" : "s3",
        "b" : "s1"
    },
    "s3":{
        "c" : "s3"
    }
}
dfa = SimpleDFA(states, alphabet, initial_state, accepting_states, transition_function)