[XRay] Fix typo in docs. NFC

Reviewers: dberris

Differential Revision: https://reviews.llvm.org/D40461

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@319047 91177308-0d34-0410-b5e6-96231b3b80d8
2 files changed