Clean up comments in function_expression_resolver.dart

In https://dart-review.googlesource.com/c/sdk/+/209240, I accidentally
included a stray "TODO" comment that only made sense in the context of
a previous patch set.  I also made a minor spelling error in a
comment.

This CL removes the stray TODO and fixes the spelling error.  There is
no functional change.

Change-Id: If1c600a579101cea1015d34d9833eb21a2b1a675
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/209400
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>
Commit-Queue: Paul Berry <paulberry@google.com>

pkg/analyzer/lib/src/dart/resolver/function_expression_resolver.dart

@@ -58,8 +58,7 @@ class FunctionExpressionResolver {
     node.visitChildren(_resolver);
     if (isFunctionDeclaration) {
       // A side effect of visiting the children is that the parameters are now
-      // in scope, so we can visit the documentation coment now.
-      // TODO(paulberry): why is a cast needed here?
+      // in scope, so we can visit the documentation comment now.
       parent.documentationComment?.accept(_resolver);
     }
     _resolve2(node);