Fix spelling mistake in rtp_rtcp.h.
Fix spelling mistake in rtp_rtcp.h.

BUG=None
R=danilchap@webrtc.org
NOTRY=True

Review-Url: https://codereview.webrtc.org/2552153003
Cr-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15435}

TimothyGu/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc,TimothyGu/libilbc,TimothyGu/libilbc,ShiftMediaProject/libilbc,ShiftMediaProject/libilbc