all: fix typos where we repeat repeat ourselves

Found with the regex `\b([A-Za-z]+) \1\b`.

Updates #cleanup

Change-Id: I4cc51784d9b6437d3d0c66b531828707f87f7fd5
Signed-off-by: Alex Chan <alexc@tailscale.com>

net/packet/header.go

@@ -45,7 +45,7 @@ type HeaderChecksummer interface {
 	Header
 
 	// WriteCheck writes the correct checksum into buf, which should
-	// be be the already-marshalled header and payload.
+	// be the already-marshalled header and payload.
 	WriteChecksum(buf []byte)
 }