Fix spelling mistake in RTP module declaration.

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

Review-Url: https://codereview.webrtc.org/2531223004
Cr-Commit-Position: refs/heads/master@{#15296}

webrtc/modules/rtp_rtcp/include/rtp_rtcp.h

@@ -129,7 +129,7 @@ class RtpRtcp : public Module {
                                        bool ipv6,
                                        uint8_t authentication_overhead = 0) = 0;
 
-  // Sets transtport overhead per packet.
+  // Sets transport overhead per packet.
   virtual void SetTransportOverhead(int transport_overhead_per_packet) = 0;
 
   // Returns max payload length, which is a combination of the configuration