Update dartdoc version to 0.15.1. (#14123)

dev/bots/docs.sh

diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index b6876ee..fd1f599 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -19,7 +19,7 @@
 fi
 
 # Install dartdoc.
-bin/cache/dart-sdk/bin/pub global activate dartdoc 0.15.0+1
+bin/cache/dart-sdk/bin/pub global activate dartdoc 0.15.1
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.