[mlir] [VectorOps] fixed typo in verifier of slice op

Reviewers: nicolasvasilache, andydavis1, rriddle

Reviewed By: nicolasvasilache, rriddle

Subscribers: merge_guards_bot, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, arpith-jacob, mgester, lucyrfox, liufengdb, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D72664