Fix variable-name typo (backport #2397) (#2399)

* Fix variable-name typo (#2397)

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 01575574

)

# Conflicts:
#	core/src/main/scala/chisel3/internal/Builder.scala

* Update Builder.scala

Co-authored-by: Tynan McAuley <tynan@galois.com>
Co-authored-by: Jiuyang Liu <liu@jiuyang.me>