[dexp] NFC: Change positional argument format

Summary:
Before:

  USAGE: dexp [options] --index-path Path to the index

After:

  USAGE: dexp [options] <INDEX FILE>

Reviewers: sammccall

Reviewed By: sammccall

Subscribers: jkorous, arphaman, kadircet, usaxena95, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D78089