Review tutorials middle

[joshwlambert]

This PR makes several minor updates the the Access epidemiological delay distributions (episodes/delays-access.Rmd), Use delay distributions in analysis (episodes/delay-functions.Rmd), Quantifying transmission (episodes/quantify-tranmission.Rmd) and Estimation of outbreak severity (episodes/severity-static.Rmd) episodes.

There are also some small edits to superspreading-estimate.Rmd and superspreading-simulate.Rmd.

This PR is part of the review of Epiverse tutorials for the upcoming Outbreak Analytics and Applied Modelling in R short course (Week 2).

Please feel free to revert any changes in this PR that you disagree with.

• Please check if the PR fulfills these requirements

• I have read the CONTRIBUTING guidelines
• A new item has been added to NEWS.md
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Checks have been run locally and pass

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

Minor textual changes.

• What is the current behavior? (You can also link to an open issue here)

N/A

• What is the new behavior (if this is a feature change)?

No new behaviour.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

• Other information:

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