[roll third_party/pigweed] pw_containers: Update FlatMap doc example so it compiles

Add a missing template argument for map size

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/156650

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 376018c32f5f57b..b3c696eae268a2c
Roller-URL: https://ci.chromium.org/b/8775210244096951601
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: Ic5f766d4d0e607e71f1245e7d5d7986a28f04fea
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/156717
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 376018c..b3c696e 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 376018c32f5f57b809f2114dc7dff2066ba3b5a2
+Subproject commit b3c696eae268a2cda2fd6fdac04328087da0b630