Change variable in README according to the PEP8

[vezeli]

Black is constantly aiming to follow PEP 8 and this is nicely put in the README. However, PEP 8 also suggests

Never use the characters 'l' (lowercase letter el), 'O' (uppercase letter oh), or 'I' (uppercase letter eye) as single character variable names.

For this reason I have made a micro alteration and changed the variable letter l in an example to another variable in the spirit of PEP 8.

[zsol]

Thanks!