[analyzer] Fix incorrect references in InterfaceItemRequirements docs.

The documentation was incorrectly referring to
`InstanceElementImpl.constructors` and
`InstanceElementImpl.getNamedConstructor`. These element do not exist,
since they are declared in the class `InterfaceElementImpl`, not
`InstanceElementImpl`.

This is a step towards enabling the `comment_references` lint in the
analyzer codebase.

Change-Id: I6a6a6964d0e005573941f53e47a624afc67aec2f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/450722
Auto-Submit: Paul Berry <[email protected]>
Reviewed-by: Konstantin Shcheglov <[email protected]>
Commit-Queue: Konstantin Shcheglov <[email protected]>

Author: stereotype441

pkg/analyzer/lib/src/fine/requirements.dart

@@ -289,10 +289,10 @@ class InterfaceItemRequirements {
   /// The value of `hasNonFinalField`, if it was requested.
   bool? hasNonFinalField;
 
-  /// Set if [InstanceElementImpl.constructors] is invoked.
+  /// Set if [InterfaceElementImpl.constructors] is invoked.
   ManifestItemIdList? allConstructors;
 
-  /// Requested with [InstanceElementImpl.getNamedConstructor].
+  /// Requested with [InterfaceElementImpl.getNamedConstructor].
   final Map<LookupName, ManifestItemId?> requestedConstructors;
 
   /// These are "methods" in wide meaning: methods, getters, setters.