Fix typo which prevents finding templates

[Bisaloo]

Follow up from #2556.

This triggers an error in template packages that provide template elements which don't exist in pkgdown, such as the carpentries/varnish package.

Error in `purrr::map_chr()`:
ℹ In index: 5.
Caused by error in `map_()`:
! Can't find template for "content-chapter".
---
Backtrace:
    ▆
 1. └─sandpaper::build_lesson()
 2.   └─sandpaper:::build_site(...)
 3.     └─sandpaper::build_episode_html(...)
 4.       └─sandpaper:::build_html(...)
 5.         └─pkgdown::render_page(...)
 6.           └─pkgdown:::render_page_html(pkg, name = name, data = data, depth = depth)
 7.             └─purrr::map_chr(pieces, find_template, name = name, pkg = pkg)
 8.               └─purrr:::map_("character", .x, .f, ..., .progress = .progress)

It solves the issue locally and seems correct based on the fact docs indeed say template.package.

With h/t to @froggleston for pinpointing where the issue may come from.

[hadley]

Oops! Thanks for the fix.