Remove backticks in Spark error message

[juliasilge]

The use of backticks in abort() here and the following lines are causing errors on Spark, shown in tidymodels/extratests#12:

parsnip/R/descriptors.R

Line 253 in e9bfce0

.x <- function() abort("Descriptor `.x()` not defined for Spark.")

To clarify, the error message is causing a failure. 😩

[github-actions]

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