Fix minor typos in comments (#5183)

mypy_extensions.py

diff --git a/mypy_extensions.py b/mypy_extensions.py
index c711e00..056b1bf 100644
--- a/mypy_extensions.py
+++ b/mypy_extensions.py
@@ -46,7 +46,7 @@
         # This method is called directly when TypedDict is subclassed,
         # or via _typeddict_new when TypedDict is instantiated. This way
         # TypedDict supports all three syntaxes described in its docstring.
-        # Subclasses and instanes of TypedDict return actual dictionaries
+        # Subclasses and instances of TypedDict return actual dictionaries
         # via _dict_new.
         ns['__new__'] = _typeddict_new if name == 'TypedDict' else _dict_new
         tp_dict = super(_TypedDictMeta, cls).__new__(cls, name, (dict,), ns)