Fix warning for multi line comment in QUIC.

Removed tailing \ character from the example usage to
make GCC to forget the multi-line comment warning.

BUG=410777
TEST=build with gcc

Review URL: https://codereview.chromium.org/534383003

Cr-Commit-Position: refs/heads/master@{#293334}

net/tools/quic/quic_client_bin.cc

@@ -6,7 +6,7 @@
 // Connects to a host using QUIC, and sends requests to the provided URLS.
 //
 // Example usage:
-//  quic_client --address=127.0.0.1 --port=6122 --hostname=www.google.com \
+//  quic_client --address=127.0.0.1 --port=6122 --hostname=www.google.com
 //      http://www.google.com/index.html http://www.google.com/favicon.ico
 
 #include <iostream>