Turns out we shouldn't call it uniqueIdentifier (#3234)

http://www.ca.com/us/services-support/ca-support/ca-support-online/knowledge-base-articles.tec465360.html
diff --git a/docs/x509/reference.rst b/docs/x509/reference.rst
index eda0581..bc408ae 100644
--- a/docs/x509/reference.rst
+++ b/docs/x509/reference.rst
@@ -2306,7 +2306,7 @@
 
         Corresponds to the dotted string ``"2.5.4.44"``.
 
-    .. attribute:: UNIQUE_IDENTIFIER
+    .. attribute:: X500_UNIQUE_IDENTIFIER
 
         .. versionadded:: 1.6