Spelling and minor changes (#12122)

doc/en/example/customdirectory/conftest.py

@@ -21,7 +21,7 @@ class ManifestDirectory(pytest.Directory):
 
 @pytest.hookimpl
 def pytest_collect_directory(path, parent):
-    # Use our custom collector for directories containing a `mainfest.json` file.
+    # Use our custom collector for directories containing a `manifest.json` file.
     if path.joinpath("manifest.json").is_file():
         return ManifestDirectory.from_parent(parent=parent, path=path)
     # Otherwise fallback to the standard behavior.