address roxygen2 "unavailable topic" warnings

[simonpcouch]

@hfrick pointed out we were seeing:

==> devtools::document(roclets = c('rd', 'collate', 'namespace'))

ℹ Updating parsnip documentation
ℹ Loading parsnip
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`topic-quosure`
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`!!`
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`topic-quosure`
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`!!`
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`topic-quosure`
Warning: [TEMPLATE:0] @details refers to unavailable topic rlang::`!!`

following #918. Turns out we don't need to backtick here!

> roxygen2:::find_topic_in_package("rlang", "`!!`")
[1] NA
> roxygen2:::find_topic_in_package("rlang", "!!")
[1] "injection-operator"

[github-actions]

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