Add semi-colon to stereo=1 param appended to SDP in workaround code

    The draft for RTP Payload Format for Opus Speech and Audio Codec (http://tools.ietf.org/html/draft-spittka-payload-rtp-opus-03)
    says that the fmtp params should be separated with a semi-colon
    This 1 character patch adds aforementioned semi-colon to make the SDP compatible with other parsers following this draft.