[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

Author: kirillbobyrev

clang-tools-extra/clangd/index/dex/dexp/Dexp.cpp

@@ -26,8 +26,7 @@ namespace clang {
 namespace clangd {
 namespace {
 
-llvm::cl::opt<std::string> IndexPath("index-path",
-                                     llvm::cl::desc("Path to the index"),
+llvm::cl::opt<std::string> IndexPath(llvm::cl::desc("<INDEX FILE>"),
                                      llvm::cl::Positional, llvm::cl::Required);
 
 llvm::cl::opt<std::string>