fix typo in sip_desktop_sharing.py

darcs-hash:20090429145521-b2873-9fe001cbdfe16aa812f3d4fa4ee55f17363c8168
sip_session2: remove SDPNegotiationError from BORING_EXCEPTIONS as it's subclass of InvitationError

darcs-hash:20090429155313-b2873-89e4a18dcf759061a195aeecdc9c0813c84415ff