[roll third_party/pigweed] pw_env_setup: Upgrade sphinx-design

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/168339

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: f20703f8eac6ab8..b8a991a6af0e109
Roller-URL: https://ci.chromium.org/b/8771925375315016305
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: If2689d82365d49c0dc399791dd0831beac7a9e1c
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/168393
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index f20703f..b8a991a 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit f20703f8eac6ab8164a2c4361a8544e704830e41
+Subproject commit b8a991a6af0e109710e65dc21460ec9d23b397d5