[build] Update rust configs path

Change-Id: I8bde199b90c3cd3314e54d7007b7dd1b9cb2320f

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index 356b38e..f9d6869 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -13,7 +13,7 @@
   name = "quiche"
   edition = "2018"
   configs -= [
-    "//build/config:rust_2018_idioms",
+    "//build/config/rust:2018_idioms",
     "//build/config:werror",
   ]
   configs += [ ":ignore_rust_warnings" ]