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-Original-Commit-Position: 972c6d2dc6dd5efdad1377c0d224e03eb8f276f7@{#15435}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 406616fc6c25f894b636f80f414a6faac7055820

Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk,Alkalyne/webrtctrunk