Fix incorrect weave setting file path

The weave setting path is incorrect due to duplicated prefix,
tinker the GetPath method a bit to make it work.

Change-Id: Icb88c29d7ce9836080814d615959eacb5e38feae
Reviewed-on: https://weave-review.googlesource.com/2470
Reviewed-by: Vitaly Buka <vitalybuka@google.com>

examples/provider/file_config_store.cc

diff --git a/examples/provider/file_config_store.cc b/examples/provider/file_config_store.cc
index a6c2e60..b215023 100644
--- a/examples/provider/file_config_store.cc
+++ b/examples/provider/file_config_store.cc
@@ -26,7 +26,7 @@
 
 std::string FileConfigStore::GetPath(const std::string& name) const {
   std::string path{kSettingsDir};
-  path += path + "weave_settings_" + model_id_;
+  path += "weave_settings_" + model_id_;
   if (!name.empty())
     path += "_" + name;
   return path + ".json";