address uninformative error in C5.0 tests

[simonpcouch]

parsnip/tests/testthat/_snaps/boost_tree_C5.0.md

Lines 3 to 8 in 4562458

	Code
	res <- fit(lc_basic, funded_amnt ~ term, data = lending_club, engine = "C5.0",
	control = ctrl)
	Condition
	Error in `glue()`:
	! Expecting '}'

Noticed this in the transition to snapshot testing in #1190.

[github-actions]

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