[mlir] NFC: fix trivial typo in documents

Reviewers: mravishankar, antiagainst, nicolasvasilache, herhut, aartbik, mehdi_amini, bondhugula

Reviewed By: mehdi_amini, bondhugula

Subscribers: bondhugula, jdoerfert, mehdi_amini, rriddle, jpienaar, burmako, shauheen, antiagainst, nicolasvasilache, csigg, arpith-jacob, mgester, lucyrfox, aartbik, liufengdb, Joonsoo, bader, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76993