fix some issues in Chapter 6 Tutorial.ipynb

[MahmoudHamdy00]

fix issues like

• 'Graph' object has no attribute 'add_cycle'
• 'Graph' object has no attribute 'node'

[filmenczer]

Is this due to a change in NetworkX? In that case, would the change break backward compatibility? @clayadavis what do you think?

[clayadavis]

This is a good change and I'll merge the PR. I do see one other instance of G.add_cycle in Tutorial 2 that should probably be fixed too.

This is because of a change in NetworkX removing some old syntax. The concern isn't with backwards compatibility because both syntaxes worked for a while, my concern is whether or not we used the old syntax in the book.

[filmenczer]

I did not find any use of .add_cycle in the book.