[mlir] [VectorOps] fixed minor typos in vector ops doc

Summary:
Rationale:
Some examples were using "offsets : [0, 2]" syntax which
should use a "=" instead. The same examples were referring
to the integer attribute array as k-dimensional, which is
a bit confusing (it is 1-dimensional, with k elements).
Changed to "k-sized".

Reviewers: nicolasvasilache, andydavis1, ftynse

Reviewed By: nicolasvasilache

Subscribers: 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/D73293