Merge pull request #2 from alex/patch-1

Slight cleanup to teh docs
diff --git a/docs/x509.rst b/docs/x509.rst
index 50790b9..e4b5e17 100644
--- a/docs/x509.rst
+++ b/docs/x509.rst
@@ -93,8 +93,9 @@
         The certificate version as an enumeration. Version 3 certificates are
         the latest version and also the only type you should see in practice.
 
-        :raises cryptography.x509.InvalidVersion: If the version is
-            not valid.
+        :raises cryptography.x509.InvalidVersion: If the version in the
+            certificate is not a known
+            :class:`X.509 version <cryptography.x509.Version>`.
 
         .. doctest::