Replace twiddle with tilde

[rok-cesnovar]

Summary

In pedantic mode docs, we use the word twiddle instead of tilde. I know they are considered synonyms, but given that we used tilde everywhere else (parser, docs, etc), it should be tilde here as well.

Copyright and Licensing

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Rok Češnovar

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• Code: BSD 3-clause (https://opensource.org/licenses/BSD-3-Clause)
• Documentation: CC-BY 4.0 (https://creativecommons.org/licenses/by/4.0/)