[mlir] NFC: Fix trivial typo in comment

Summary: Also, an exercise to merge this into the master myself after a reviewer gives LGTM.

Reviewers: nicolasvasilache, mehdi_amini

Reviewed By: mehdi_amini

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73432