Bump dartdoc to 0.28.3+2 (#31148)

This version of dartdoc properly escapes fenced code block "info strings."
diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index 5dab299..79cd279 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -104,7 +104,7 @@
 fi
 
 # Install and activate dartdoc.
-"$PUB" global activate dartdoc 0.28.3+1
+"$PUB" global activate dartdoc 0.28.3+2
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.