fix gtfs_realtime_version in FeedHeader

[JNaftali]

in gtfs-realtime/proto/gtfs-realtime.proto the FeedHeader message was misconfigured. gtfs_realtime_version was set = to 2 (the version number) instead of 1 (the field number)

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[JNaftali]

submitted cla

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CLAs look good, thanks!

[barbeau]

Whoops! Thanks for catching that @JNaftali. LGTM.