Fix docstring example reference

[pouwerkerk]

There appears to be a very small typo in the docstrings for Duration.normalize(). The docstring mentions negative values, but points to an example about excessive values, whereas the third example appears to exemplify the point made in the docstring.

[linux-foundation-easycla]

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• ✅ login: pouwerkerk / name: Pieter Ouwerkerk (1ae95fa)

[icambron]

thanks