Minor typo in quickstart.ipynb

[emptymalei]

In the first example, t_span in the lightning module should be self.t_span

t_span -> self.t_span

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (1e8979a) 66.15% compared to head (07ca60e) 65.30%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #207 +/- ## ========================================== - Coverage 66.15% 65.30% -0.86% ========================================== Files 29 29 Lines 2074 2101 +27 ========================================== Hits 1372 1372 - Misses 702 729 +27 ```

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[Zymrael]

Thanks!