Fixed mismatched parameter documentation
diff --git a/OpenSSL/crypto.py b/OpenSSL/crypto.py
index 7b2ce07..132d98d 100644
--- a/OpenSSL/crypto.py
+++ b/OpenSSL/crypto.py
@@ -627,9 +627,9 @@
         """
         Initializes an X509 extension.
 
-        :param typename: The name of the type of extension to create. See
+        :param type_name: The name of the type of extension to create. See
             http://openssl.org/docs/apps/x509v3_config.html#STANDARD_EXTENSIONS
-        :type typename: :py:data:`bytes`
+        :type type_name: :py:data:`bytes`
 
         :param bool critical: A flag indicating whether this is a critical
             extension.