[roll third_party/pigweed] docs: Mention command for updating Py dep hashes

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/177799

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: e6aa95bf08fba2b..7eb9df1b37c6a76
Roller-URL: https://ci.chromium.org/b/8766224622694560001
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ie8128145a95e87605d3fa1ed7783c9e10ffc05d3
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/kudzu/+/177803
Bot-Commit: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
Commit-Queue: Pigweed Roller <pigweed-roller@pigweed-service-accounts.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index e6aa95b..7eb9df1 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit e6aa95bf08fba2bd9120b61d94e77d4d2b60c721
+Subproject commit 7eb9df1b37c6a76c54e9571f960dc58e317ed434