[roll third_party/pigweed] docs: Clarify commit message footers

Both "Fixed" and "Fixes" are supported (for Buganizer; Monorail only
supports Fixed, see b/206842834). This section used to be somewhat
confusing, because it mentioned only "Fixed" in the body, but used
"Fixes" in the example.

Original-Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/112317

https://pigweed.googlesource.com/pigweed/pigweed
third_party/pigweed Rolled-Commits: 99c5463d2c830d2..edf53b2f0cd9384
Roller-URL: https://ci.chromium.org/b/8801753450122131745
GitWatcher: ignore
CQ-Do-Not-Cancel-Tryjobs: true
Change-Id: I42d9c0cdc3a7b7ab62ecd045872e58fa9ce0e7e6
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/experimental/+/112435
Commit-Queue: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
Bot-Commit: Pigweed Integration Roller <pigweed-integration-roller@pigweed.google.com.iam.gserviceaccount.com>
diff --git a/third_party/pigweed b/third_party/pigweed
index 99c5463..edf53b2 160000
--- a/third_party/pigweed
+++ b/third_party/pigweed
@@ -1 +1 @@
-Subproject commit 99c5463d2c830d2bb05136c76081f3166a6c68ab
+Subproject commit edf53b2f0cd9384e3f15a0cf4777c2399e96ee94