Fix typos in TypeIs docstring (#524)

Author: hamdanal

src/typing_extensions.py

@@ -2141,7 +2141,7 @@ def TypeIs(self, parameters):
 
         1. The return value is a boolean.
         2. If the return value is ``True``, the type of its argument
-        is the intersection of the type inside ``TypeGuard`` and the argument's
+        is the intersection of the type inside ``TypeIs`` and the argument's
         previously known type.
 
         For example::
@@ -2189,7 +2189,7 @@ def __getitem__(self, parameters):
 
         1. The return value is a boolean.
         2. If the return value is ``True``, the type of its argument
-        is the intersection of the type inside ``TypeGuard`` and the argument's
+        is the intersection of the type inside ``TypeIs`` and the argument's
         previously known type.
 
         For example::