Merge pull request #149 from jeffswartz/sip-custom-headers-docs-update

Correction to SIP docs

Author: superdiana

opentok/opentok.py

@@ -842,8 +842,7 @@ def dial(self, session_id, token, sip_uri, options=[]):
             as the caller
 
             Dictionary 'headers': Defines custom headers to be added to the SIP INVITE request
-            initiated from OpenTok to the SIP platform. Each of the custom headers must
-            start with the "X-" prefix, or the call will result in a Bad Request (400) response
+            initiated from OpenTok to the SIP platform
 
             Dictionary 'auth': Contains the username and password to be used in the the SIP
             INVITE request for HTTP digest authentication, if it is required by the SIP platform