[mlir] [VectorOps] refined description of vector.contract

Summary:
A few details were missing in the description. These
changes makes the documented code "compile".

Reviewers: nicolasvasilache, andydavis1

Reviewed By: nicolasvasilache, andydavis1

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

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D73923