Fix markup
Thanks, @hynek!
diff --git a/OpenSSL/crypto.py b/OpenSSL/crypto.py
index 3b8abeb..d642c6c 100644
--- a/OpenSSL/crypto.py
+++ b/OpenSSL/crypto.py
@@ -1487,7 +1487,7 @@
:param X509 cert: The certificate to add to this store.
:raises TypeError: If the certificate is not an :py:class:`X509`.
:raises Error: If OpenSSL was unhappy with your certificate.
- :return: py:data:`None` if the certificate was added successfully.
+ :return: :py:data:`None` if the certificate was added successfully.
"""
if not isinstance(cert, X509):
raise TypeError()