Removes comma from end of line

[copycatchiller]

I believe there is an accidental comma at the end of the assert statement, which will cause a syntax error.

[julienr]

Thanks for looking at this.

The assert is written as

assert difficulty >= 1, "Difficulty of 0 is not possible"

which is correct syntax. The comma at the end of line in the .ipynb file is because the notebook is stored as JSON, so it's the JSON separator, but this is fine as well.

The notebook works fine on my machine (python 3.6.2, jupyter 4.3.0), maybe you have another version ?