Fix issue in python sample source in Tutorials.md
Fix issue in python sample source in Tutorials.md

Updated python sample source in Tutorials to make it work. Before this change problem was:

```
C:\Users\user>python mermaid.py
Traceback (most recent call last):
  File "mermaid.py", line 3, in <module>
    graphbytes = graph.encode("ascii")
NameError: name 'graph' is not defined
```
knsv/mermaid,knsv/mermaid