Fix CheckWidths error message for uninferred width (backport #2196) (#2197)

* Fix CheckWidths error message for uninferred width (#2196)

Looks like a typo/auto-merge hiccup.

(cherry picked from commit adc2ad9a

)

# Conflicts:
#	src/main/scala/firrtl/passes/CheckWidths.scala

* fixup! Fix CheckWidths error message for uninferred width (#2196)

Co-authored-by: Fabian Schuiki <fabian@schuiki.ch>
Co-authored-by: Schuyler Eldridge <schuyler.eldridge@sifive.com>