Bump dartdoc to 0.28.3+2 (#31148)

This version of dartdoc properly escapes fenced code block "info strings."

dev/bots/docs.sh

@@ -104,7 +104,7 @@ if [[ -d "$FLUTTER_PUB_CACHE" ]]; then
 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.