fix formatting in prompt_missing_implementation()

[simonpcouch]

cli_inform() handles nested {} a bit differently than cli_warn() and cli_abort().

library(cli)

exclamation <- "Darn!"
msg <- c("!" = "That didn't work. {{exclamation}}")

cli_inform(msg)
#> ! That didn't work. Darn!
cli_warn(msg)
#> Warning: ! That didn't work. {exclamation}
cli_abort(msg)
#> Error:
#> ! That didn't work. {exclamation}

prompt_missing_implementation() made use of that off-label behavior in cli_inform(), and this caused some wonkiness when called with the warn or abort prompts and a non-missing engine:

library(parsnip)

# ope -- note "using the `{engine}` engine."
cubist_rules(engine = "Cubist") %>% fit(mpg ~., data = mtcars)
#> Error in `fit()`:
#> ! parsnip could not locate an implementation for `cubist_rules` model
#>   specifications using the `{engine}` engine.
#> ℹ The parsnip extension package rules implements support for this
#>   specification.
#> ℹ Please install (if needed) and load to continue.

^{Created on 2022-09-08 by the reprex package (v2.0.1)}

With this PR:

library(parsnip)

cubist_rules(engine = "Cubist") %>% fit(mpg ~., data = mtcars)
#> Error in `fit()`:
#> ! parsnip could not locate an implementation for `cubist_rules` model
#>   specifications using the `Cubist` engine.
#> ℹ The parsnip extension package rules implements support for this
#>   specification.
#> ℹ Please install (if needed) and load to continue.

cubist_rules(engine = "Cubist")
#> ! parsnip could not locate an implementation for `cubist_rules` model
#>   specifications using the `Cubist` engine.
#> ℹ The parsnip extension package rules implements support for this
#>   specification.
#> ℹ Please install (if needed) and load to continue.
#> Cubist Model Specification (regression)
#> 
#> Computational engine: Cubist

^{Created on 2022-09-08 by the reprex package (v2.0.1)}

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