[clangd] Tweak the no-index error message for rename, NFC.

[hokein]

Summary

The current error message doesn't fit well for cross-file rename.

[ilya-biryukov]

LGTM

clang-tools-extra/clangd/refactor/Rename.cpp

 }
 
 llvm::Error makeError(ReasonToReject Reason) {
   auto Message = [](ReasonToReject Reason) {
     switch (Reason) {
     case ReasonToReject::NoSymbolFound:
       return "there is no symbol at the given location";
     case ReasonToReject::NoIndexProvided:
-      return "symbol may be used in other files (no index available)";
+      return "no index provided";
     case ReasonToReject::UsedOutsideFile:
       return "the symbol is used outside main file";
     case ReasonToReject::NonIndexable:
       return "symbol may be used in other files (not eligible for indexing)";
     case ReasonToReject::UnsupportedSymbol:
       return "symbol is not a supported kind (e.g. namespace, macro)";
     case AmbiguousSymbol:
       return "there are multiple symbols at the given location";