[build] Update BUILD.gn after tag merge

Bug: 86678

Change-Id: Id6d3afd7e1750bd75f8a46f2c9f092f976dbe376
Reviewed-on: https://fuchsia-review.googlesource.com/c/third_party/pyyaml/+/594548
Reviewed-by: Shai Barack <shayba@google.com>

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index c7e5ef6..37298ef 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -5,7 +5,7 @@
 import("//build/python/python_library.gni")
 
 python_library("yaml") {
-  source_root = "lib3/yaml"
+  source_root = "lib/yaml"
   sources = [
     "__init__.py",
     "composer.py",