analyzer: little rename of LibraryElement2Extensions

I forgot we name an extension with "Extension", not "Extensions".

Luckily, while in the public API, this was never included in a release
of the analyzer, so it doesn't need to be shipped in a major version
release.

Change-Id: Ic66c13bac3d5b246813af6e1770a0ef6543827fd
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/420421
Reviewed-by: Paul Berry <[email protected]>
Auto-Submit: Samuel Rawlins <[email protected]>
Commit-Queue: Paul Berry <[email protected]>

Author: srawlins

pkg/analyzer/api.txt

@@ -5318,7 +5318,7 @@ package:analyzer/src/task/api/model.dart:
 package:analyzer/src/workspace/workspace.dart:
   Workspace (non-public)
 package:analyzer/utilities/extensions/element.dart:
-  LibraryElement2Extensions (extension on LibraryElement2):
+  LibraryElement2Extension (extension on LibraryElement2):
     exportedExtensions (getter: Iterable<ExtensionElement2>)
 package:analyzer/utilities/extensions/uri.dart:
   UriExtension (extension on Uri):

pkg/analyzer/lib/utilities/extensions/element.dart

@@ -4,7 +4,7 @@
 
 import 'package:analyzer/dart/element/element2.dart';
 
-extension LibraryElement2Extensions on LibraryElement2 {
+extension LibraryElement2Extension on LibraryElement2 {
   /// All extensions exported from this library.
   Iterable<ExtensionElement2> get exportedExtensions {
     return exportNamespace.definedNames2.values.whereType();