Update dartdoc to 0.20.4. (#21131)

diff --git a/dev/bots/docs.sh b/dev/bots/docs.sh
index 04f76d2..ee4acb5 100755
--- a/dev/bots/docs.sh
+++ b/dev/bots/docs.sh
@@ -63,7 +63,7 @@
 fi
 
 # Install and activate dartdoc.
-"$PUB" global activate dartdoc 0.20.3
+"$PUB" global activate dartdoc 0.20.4
 
 # This script generates a unified doc set, and creates
 # a custom index.html, placing everything into dev/docs/doc.