Update dartdoc version to 0.26.1.

Release notes:  https://github.com/dart-lang/dartdoc/releases/tag/v0.26.1



Change-Id: If9aaea8c10537103433a1bc586edaf113a89408c
Reviewed-on: https://dart-review.googlesource.com/c/87261
Reviewed-by: Paul Berry <paulberry@google.com>
Commit-Queue: Janice Collins <jcollins@google.com>
diff --git a/DEPS b/DEPS
index db23de0..9dcdf62 100644
--- a/DEPS
+++ b/DEPS
@@ -81,7 +81,7 @@
   # For more details, see https://github.com/dart-lang/sdk/issues/30164
   "dart_style_tag": "1.2.2",  # Please see the note above before updating.
 
-  "dartdoc_tag" : "v0.26.0",
+  "dartdoc_tag" : "v0.26.1",
   "file_rev": "515ed1dd48740ab14b625de1be464cb2bca4fefd",  # 5.0.6
   "fixnum_tag": "0.10.9",
   "func_rev": "25eec48146a58967d75330075ab376b3838b18a8",