[mlir] NFC: Fix trivial typos in documents

Fix trivial typos

Reviewers: mravishankar, antiagainst, ftynse

Reviewed By: ftynse

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D76347