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: refs/heads/master@{#15435}
Cr-Mirrored-From: https://chromium.googlesource.com/external/webrtc
Cr-Mirrored-Commit: 406616fc6c25f894b636f80f414a6faac7055820
1 file changed