[clangd][NFC] Update background-index command line description

Summary:
We didn't change this in D64019 just in case we revert it back.
Deleting it now.

Reviewers: hokein, sammccall

Subscribers: ilya-biryukov, MaskRay, jkorous, arphaman, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D66943

llvm-svn: 370367