Add assertCount for filesystem doc (#10041)

* Add assertCount for filesystem doc

* Update filesystem.md

---------

Co-authored-by: Taylor Otwell <taylor@laravel.com>

Author: ahmadreza1383

filesystem.md

@@ -701,6 +701,9 @@ test('albums can be uploaded', function () {
     Storage::disk('photos')->assertMissing('missing.jpg');
     Storage::disk('photos')->assertMissing(['missing.jpg', 'non-existing.jpg']);
 
+    // Assert that the number of files in a given directory matches the expected count...
+    Storage::disk('photos')->assertCount('/wallpapers', 2);
+
     // Assert that a given directory is empty...
     Storage::disk('photos')->assertDirectoryEmpty('/wallpapers');
 });
@@ -734,6 +737,9 @@ class ExampleTest extends TestCase
         Storage::disk('photos')->assertMissing('missing.jpg');
         Storage::disk('photos')->assertMissing(['missing.jpg', 'non-existing.jpg']);
 
+        // Assert that the number of files in a given directory matches the expected count...
+        Storage::disk('photos')->assertCount('/wallpapers', 2);
+
         // Assert that a given directory is empty...
         Storage::disk('photos')->assertDirectoryEmpty('/wallpapers');
     }