fix template page permissions issue

[zkamvar]

This ensures that templated files have the right permissions when appending common links to them so that 404 pages and other generated content can be built.

This will fix #479

This PR can be tested by installing it with the {pak} OR {remotes} package:

# via {pak}
# install.packages("pak")
pak::pkg_install("carpentries/sandpaper#482")

# via {remotes}
# install.packages("remotes")
remotes::install_github("carpentries/sandpaper#482")

[ocaisa]

Typo