Quickstart: Fixed training_step `t_span`

[v-i-s-h]

t_span was missing self reference

[codecov-commenter]

Codecov Report

Base: 62.53% // Head: 62.53% // No change to project coverage :thumbsup:

Coverage data is based on head (2ee33e1) compared to base (a71cdcf). Patch has no changes to coverable lines.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #176 +/- ## ======================================= Coverage 62.53% 62.53% ======================================= Files 27 27 Lines 1879 1879 ======================================= Hits 1175 1175 Misses 704 704 ``` Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DiffEqML). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=DiffEqML)

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