subprojects: Update to HEAD

Change-Id: Ifff1c16dd8944e54de89e9c95a2b7df1292486b9
Signed-off-by: William A. Kennington III <wak@google.com>
diff --git a/subprojects/fmt.wrap b/subprojects/fmt.wrap
index bd840f4..6847ae5 100644
--- a/subprojects/fmt.wrap
+++ b/subprojects/fmt.wrap
@@ -1,3 +1,3 @@
 [wrap-git]
 url = https://github.com/fmtlib/fmt
-revision = 6.2.1
+revision = HEAD