Fix a typo in AMDGPU docs

Reviewers: t-tye, arsenm

Reviewed By: arsenm

Subscribers: kzhuravl, jvesely, wdng, nhaehnle, yaxunl, dstuttard, tpr, kerbowa, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D81247