[docs] Fic grammar in ssl.SSLContect.sslsocket_class docstring

Author: PrinceNaroliya

Doc/library/ssl.rst

@@ -1898,7 +1898,8 @@ to speed up repeated connections from the same clients.
 .. attribute:: SSLContext.sslsocket_class
 
    The return type of :meth:`SSLContext.wrap_socket`, defaults to
-   :class:`SSLSocket`. The attribute can be overridden on instance of class
+   :class:`SSLSocket`. The attribute can be assigned to on instances of
+   :class:`SSLContext` or overridden on subclasses of :class:`SSLContext`
    in order to return a custom subclass of :class:`SSLSocket`.
 
    .. versionadded:: 3.7