Spelling correction for docs for cppcoreguidelines-owning-memory

Summary: There's a typo in the docs, as mentioned in the title. Please see the diff.

Reviewers: JonasToth

Subscribers: sylvestre.ledru, nemanjai, kbarton, cfe-commits

Tags: #clang-tools-extra, #clang

Differential Revision: https://reviews.llvm.org/D60050

llvm-svn: 357371