Update Pigweed branch

Change-Id: I77e0c473d363e0f5dd35f1774ef5b9a4afd5d8d7
Bug: 209
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/48020
Reviewed-by: Keir Mierle <keir@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
diff --git a/.gitmodules b/.gitmodules
index ecc3288..ed00f98 100644
--- a/.gitmodules
+++ b/.gitmodules
@@ -1,7 +1,7 @@
 [submodule "third_party/pigweed"]
 	path = third_party/pigweed
 	url = https://pigweed.googlesource.com/pigweed/pigweed
-	branch = master
+	branch = main
 [submodule "third_party/nanopb"]
 	path = third_party/nanopb
 	url = https://pigweed.googlesource.com/third_party/github/nanopb/nanopb