Fix README.Rmd typo: Correct day count

[darakhshannehal]

• Please check if the PR fulfills these requirements

• [x ] I have read the CONTRIBUTING guidelines
• [ x] A new item has been added to NEWS.md - Not applicable for a documentation-only change.
• [ x] Tests for the changes have been added (for bug fixes / features) - Not applicable for a documentation-only change.
• [x ] Docs have been added / updated (for bug fixes / features)
• [ x] Checks have been run locally and pass - Not applicable for a documentation-only change.

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

• What is the current behavior? (You can also link to an open issue here)
  The README.Rmd file contains a typo in a code example, saying '20 days the outbreak' instead of '30 days of the outbreak'.

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

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)
  No

• Other information: