Fix format specifier

Author: Michael137

lldb/source/Plugins/Language/ObjC/NSSet.cpp

@@ -395,7 +395,7 @@ lldb_private::formatters::NSSetISyntheticFrontEnd::GetIndexOfChildWithName(
       (idx < UINT32_MAX && idx >= CalculateNumChildrenIgnoringErrors()))
     return llvm::createStringError(
         "'SyntheticChildrenFrontEnd::NSSetISyntheticFrontEnd' cannot find "
-        "index of child '%s'. (idx='%d')",
+        "index of child '%s'. (idx='%" PRIu32 "')",
         name.AsCString(), idx);
   return idx;
 }