[clangd] Fix a typo in TUSchedulerTests

Reviewers: ilya-biryukov

Reviewed By: ilya-biryukov

Subscribers: javed.absar, ioeric, MaskRay, jkorous, arphaman, kadircet, cfe-commits

Differential Revision: https://reviews.llvm.org/D55312

llvm-svn: 348467