Spell check of Flutter docs (#45200)

No code changes, just comments: I spell-checked all the comments in the repo.
diff --git a/dev/bots/test.dart b/dev/bots/test.dart
index 0fb34b3..e26c955 100644
--- a/dev/bots/test.dart
+++ b/dev/bots/test.dart
@@ -961,7 +961,7 @@
 }
 
 /// If the CIRRUS_TASK_NAME environment variable exists, we use that to determine
-/// the shard and subshard (parsing it in the form shard-subshard-platform, ignoring
+/// the shard and sub-shard (parsing it in the form shard-subshard-platform, ignoring
 /// the platform).
 ///
 /// However, for local testing you can just set the SHARD and SUBSHARD