Fix initial growth

[sbfnk]

Description

This PR fixes a bug whereby the initial growth was never estimated because of a misnamed argument wrapped inside a purrr::safely call. Not a big deal probably, as this is poorly identified anyway (highly anti-correlated with the number of initial infections) and has a comparatively wide prior sd.

Initial submission checklist

• My PR is based on a package issue and I have explicitly linked it.
• I have tested my changes locally (using devtools::test() and devtools::check()).
• I have added or updated unit tests where necessary.
• I have updated the documentation if required and rebuilt docs if yes (using devtools::document()).
• I have followed the established coding standards (and checked using lintr::lint_package()).
• I have added a news item linked to this PR.

After the initial Pull Request

• I have reviewed Checks for this PR and addressed any issues as far as I am able.

[seabbs]

Not ideal. Looks good.