Issue 86: Fixes for "Why it works" vignette

Description

This PR makes some fixes to the "Why it works" vignette:

Corrects a typo found by @parksw3 https://github.com/epinowcast/primarycensoreddist/issues/83#issuecomment-2359178837
Mentions the shift-invariance assumption on delay we are making (also mentioned by @parksw3)
Reduces the width of various eqns that are overrunning.

Closes #86

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