docs: fix simple typo, iinstance -> isinstance (#774)

There is a small typo in jwt/algorithms.py.

Should read `isinstance` rather than `iinstance`.

jwt/algorithms.py

@@ -609,7 +609,7 @@ def sign(self, msg, key):
             Sign a message ``msg`` using the EdDSA private key ``key``
             :param str|bytes msg: Message to sign
             :param Ed25519PrivateKey}Ed448PrivateKey key: A :class:`.Ed25519PrivateKey`
-                or :class:`.Ed448PrivateKey` iinstance
+                or :class:`.Ed448PrivateKey` isinstance
             :return bytes signature: The signature, as bytes
             """
             msg = bytes(msg, "utf-8") if type(msg) is not bytes else msg